After mounting the body on the body posts, remove the body and align the tracers to the tires as shown
Slide the tracers under the tires as shown leaving enough room fo the body to fit in between the tracer and the tire. The cradle shape of the tracer's base will keep it centered to the tire.
Mount the body back onto the posts and push the tracer against the body. Once this is done then simply take a sharpie and trace around the tracer.
Remove the tracers and double check your marks to make sure you have a clean line to cut from. Repeat the process for the other side
Once both sides are complete, remove the body and cut out the wheel wells and you're done. It's that Simple!