Okay so I'm making a "CARS" cake and I'm making some of the cars out of gumpaste/fondant to put up against the sides of the cake so they are flat but still kinda three D. My question to you is, do I have to make them and stick them to the cake right away. Usually when I make a gumpaste figure it can just dry and I add it to the cake but since these will be flush with the cake (and this will make them a little rounded) do I need to wait to make them. Or do you think if I make them flat they will go on the cake correctly. I hope I'm making my point clear

If you want them rounded, sitting flush with the side of the cake, the you can not let them dry flat. Or to state this another way, make them and immediately place them on the cake.

You could also get a cake round the size of you cake and cut it in half. I cover mine with plastic wrap so the gumpaste isn't touching the styrofoam. Make your car and place on the round so that it dries with the curve.