Those kits are very DIY-heavy, you'll need to design a certain amount of it yourself along the way to get it together, like the body mounts.
Typically you'll mount posts to the frame and then cut small holes in the body that hold it in place with cotter pins. Where the posts are located on the model varies, as does the shape of the body and where you'll choose to support it, so the bodies usually come without any holes (unless they're being sold for a very specific model)