Attaching the Body With Body Mounting Posts: Each body mounting post has at least one small hole in it. After putting the body over the posts, a body pin -- a small paperclip-like cotter pin -- goes through the hole in the post to hold the body on the chassis.
Some RCs have body mounting posts with multiple holes. This allows you to raise and lower the body to suit different styles of racing and appearance.
When making a new body for an RC, you'll need to mark locations on the body that match up with the body mounting post locations and use a reamer to punch or cut holes in the new body.
Alternative Pins for the Body Mouns: The little body pins that hold the body on the chassis are a frequently lost item. A quick substitute can be made from a paperclip or other piece of stiff wire. To keep from losing the body clips in the first place you can tie them to a tiny zip tie or small piece of fishing line that attach to the body. That way, if the clips fall out of the body mounting posts (or when you take them out yourself) the clips aren't lost.