Once the Mission Editor opens, select "Create a New Map" in the pop-up Dialog. In the next frame, select Map type (6v6 or 12v12), then Size (this is the base environment you will start from), and Template (this is the mission you will start with). For our purposes, let's select FLO (12v12), Huge size, No Objectives. Name your map in the 'Mission Name' text box - if your text isn't showing up, simply click the black background of the mission editor, then click in the dialog box again and begin typing. Let's name this map "TestMap". You can add a mission description at this stage, but it's unnecessary - we will add a better description once our environment is built and mission set.
Now that your map is open, we should layout a space for gameplay. If you're unfamiliar with the camera controls, take some time to click and hold Left Mouse Button, Right Mouse Button, and/or both while moving the mouse around. You can also use WASD to move as you would in most FPS games. Let's make a simple box-shaped building in the middle of the map with two floors. First, click the "Place Map Objects" tab at the bottom left of the screen - this gives us access to placeable objects through a set of filters. Click on "Walls", near the bottom of the list, then you can choose a type of wall. Let's choose Cement for now.
It's important to note that Unreal uses a grid for object placement, and AA: PG's wall assets are built to a certain size so that they all match up when placed next to each other. Placing one object on top of another can move the object you're placing off of the grid, so with that in mind, we'll do our placement of objects onto a flat stretch of neutral ground, rather than placing objects on top of each other. The terrain in template maps is perfectly flat if you don't change it, so anywhere in the flat space works for this. Click on Cement Double Window, then move your cursor around the level to see the item you're going to place - place one on open neutral ground, then press Escape to get your cursor back. Repeat this process with Full Wall and Single Door.
Now click on the Full Wall, move it into place near the center of the map by clicking and dragging the little arrows of the widget that pops up. Hold Alt, and drag one of those arrows to duplicate the wall piece, and place that wall piece next to the first one; it's best to 'push' your copy from where the widget sits on the asset, rather than 'pull' it; this will make rotating at corners much easier. Do this until you have a wall that is 5 wall pieces in length - note that you must release the Alt key in-between each duplication.
Select your fifth tile, then Hit Spacebar; you now have the rotation, rather than translation, widget. Click the Blue arc and drag to rotate the wall along it's Z-axis, forming a 90-degree corner. Hit Spacebar again to get the translation widget, and duplicate the wall until it's again 5 wall pieces long. Rotate the fifth piece as before, then repeat this process until you have a box.
Now, let's make a second floor quickly and easily. Hold Ctrl and click each wall piece that makes up the box - each one should be a lilac color, showing that it's currently selected. Hold Alt, and click+drag the Blue arrow on the translation widget until you get a second floor. Hit Escape to de-select these wall pieces.
Now it's time to give our box some windows and doors. First, let's put two doors on the first floor - delete two wall pieces by selecting them, then hitting Delete. Move the Door piece you placed earlier into one of the wall spaces you just created, then duplicate and place a second one. Repeat this process of deleting a few walls and replacing them with windows. Note that some wall types, such as Cement, must have the same type of side facing the same direction as the pieces it's next to; in simpler terms, the side with bolts (in the case of Cement walls) should face out for each piece and the painted side faces in, or else the pieces don't neatly line up on the grid.
Next, our box building needs floors and a roof - find "Floors and Ceilings" in the filter list, and select "Floors". If you want to remove the Cement Wall pieces from your active list, open "Active Filters" at the top of the filters list and uncheck "Cement". Select and place one "Metal Floor" on open, neutral ground, then duplicate as above to get a 4-by-4 square of floor tiles. Move that 4-by-4 square into your box building to form the first floor, then duplicate up (Alt+drag the blue arrow) to make the second floor and finally the roof.
There's currently no way to get to the second floor; we need stairs. In the Filters list, open "Structures" and check "Stairs". Now let's cut out some of the second floor to allow space for the stairs - pick two second-floor tiles that are in the middle of a wall, and delete them. Then, outside the building, place a set of stairs - "Metal Stairs 2" works well for this. Move the Stairs into position as best you can leading from the first floor up to the second.
Finally, our building is going to be dark if we don't add any lights inside. Open "Lights", then "Mounted" and select a light - "Bulb Fixture" works well here. Place one inside the room, and set your grid - the slider at the top of your Mission Editor window reading "64x64" or similar - to 2 or 4, by using the +/- keys or by moving the slider. Rotate and place one light on the first floor, then duplicate it for a second light to place on the second floor.
Once you've placed your lights, it's a good idea to Build lighting to see how they'll really look in-game; do this by selecting "Build" in the tools menu or pressing Ctrl+B. Check "Build Geometry", "Build Lighting", and "Build Cover" - selecting "High Quality Lighting" is unnecessary at this stage. Click the "Build" button to begin the process, and wait the few seconds to minutes that it takes to build. Once the build dialog says "Build Completed Successfully!", click the close button to resume adding objects to your level.
Now let's add a mission - we'll go with something simple, an Activate mission with two objectives. Open "Gameplay", then "Objectives", and select the "Activate" objective (with the electrical plug icon). Rather than placing on neutral ground (since these don't have to line up with anything in particular), you can place one each directly into the first and second floor rooms, but for ease of use, make sure to place them in opposite corners of the building. Let's make the first floor X, and the second floor Y - do this by selecting the first floor objective and opening "Object properties", the tab on the top-right of the Mission Editor window. Set Objective Name to "X", "Only Capture Once" to "On", and "Allowed to Capture" to "Team 1". Repeat this process with the upstairs objective, changing the name to "Y".
Now we need to hook up our mission so that when Attack takes both objectives, they win. First, let's save our work by selecting "Save" in the Map Menu. Then open the Tools Menu and select Objective Editor, or press Ctrl+Shift+O. A new box should show up labeled "Mission Flow". It defaults to Team 0's win condition, the team that will start labeled "Attack" - you can verify this by looking at the Objective Order Pane, which should have a red dot next to it. Drag from Attack's Deployment point - the big red circle on the overhead map - to each of the objectives. You should see arrows connecting them. Now Hold Ctrl and click X, then click and drag from Y to the big blue circle labeled "B" - you should see a new node with an ampersand symbol in the middle. At this point your mission flow essentially reads "Players spawning on Team 0 can activate X and Y; when both are activated, they win." If you were to drag from X and y to the big blue circle individually, that would mean that activating X OR Y would trigger a Win for Team 0. Close the Objective editor by clicking on the small grey X.
At this point you could continue adding walls, tables, stairs, ladders, lights, etc - but let's test our mission first by saving our work, then right-clicking in the world and selecting "Play Level". You will spawn in on Team 0 (The 'Red' set of spawn points in one corner of the map) in warm-up mode, meaning the round immediately begins and the timer is frozen. Run into your building and try activating both objectives; they should appear gray if you left "Start Controlled by" as Neutral, and you should be able to activate both of them; once activated they should disable themselves (hiding their HUD icon), and once both are activated you should successfully trigger a round win.
To publish the map, we would first want to build High-Quality Lighting, then select "Publish" from the Tools menu. The resulting dialog box will let you change the name of your map, add or change your mission description, and set who on the Steam Workshop can see your map (Everyone, Friends, or Hidden, which means only you can see it). You can also Right-Click and Hold in the window on the right side to change the preview image camera placement - this image will be uploaded with your map and shown in the Steam Workshop. Once you're satisfied with what you've set up here, simply press "OK" and wait for the publish to finish. If it fails with an error, wait a few minutes and check your Steam Workshop to see if it successfully published; if not, often you can simply try it again until it works.
At this point, you should have a working Activate map. You could continue building this space out with more structures and objects, or altering the mission, or you could simply start over in a new map. The best way to continue learning is to try things - so long as you save often, you can and should try anything you can think of. And once you're comfortable working with the Mission Editor, don't be afraid to dive into the fully-featured Advanced Editor - it's nearly identical to the professional tools used to create Unreal Engine-based games and the Unreal Development Kit, and there are tons of tutorials available to guide you there. The Advanced Editor offers many more possibilities for creating compelling original content- whether through new actor types, new objects and tools, or more settings to play with.