how do you design a cover for your game?

You can't

Heh sorry I meant how do you make something like this

Oh. Ok. If you want a background (like the grass on the title of slime simulator) then you will put BGFILL. After that, put the starting x,starting y, ending x, ending y, and the tile id. To find the tile id, goto the smile tool and click map. Search the tiles on the right, and select the tile you want. At the top of the box, you will see the id number. Example: &H0003