When in edit mode in forge build a stack of crates or whatever until it's as high as you want, then build a 'solid object' ontop of the stack, these are objects you can't move by meleeing them, bridges, walls, large crates, ect. After the 'solid object' is sitting still on the stack you can delete the stack from under it, it will stay in the air floating. A good technic is to make some floating bridges next to each other, then you have a floating platform to build stuff on or whatever you want, have fun.