Practice.
Create a basic Night or Day sky that would be half the height of your window layout. Duplicate it if it is a sprite object (Repeat duplication) and place it for the length of the level you want. If you are using a Tiled object, you can just stretch it out.
For the ground, I’ve primarily created a rectangle (practice making grass or a street). Make sure it is around 1/3 of the Window Layout Dimensions you set. Of course, you can make it bigger, that is up to you. If it is a sprite object, you’ll have to duplicate and place until you fill the layout (not window size) of how long you want to level. If it is Tiled object, then you can just click on an end and stretch it out.
Then start creating buildings, trees, objects and place them appropriately (buildings and so forth will take up some of your sky).
Right click on the objects to check the Z order as to what is in front of what.
If you are making a “fake” 3D (2.5) style game, Then your character will have the 8 direction behavior and can go all over the screen such as walk on the sky. So you will need to create “invisible” solid objects to place high or low enough to prevent them from walking anywhere. There is another way to do it in events instead of adding solid objects by using Viewports and the exact x y axis of where you want your player to not walk. I can’t remember off hand though.
If you are making a platform, you just need to add a solid “invisible” line along the ground and give the player platform behavior instead of 8 directional.
Hope that helps.