Milling foam parts is fairly easy, once you learn the basics... Depending on the type of CNC you have, body parts are usualy 1 surface / 1 operation relief cutting, like sign making on a Shapeko... You layout the part on a flat surface in you CAD software, drop the edges to remove undercuts, set the Z axis to zero on the flat surface, and you're ready to go. More complex shapes will require multiple setups are piecing seperate parts together for your final molding.

You can run foam much like wood... Due to the density, the CNC can run at higher feed speeds, just watch your spindle speed. You also want to make sure your bits are as sharp as possible, as dull edges can rip & tear instead of making a clean cut.

the wrong speeds can easily melt & burn foam, It'll take experimenting & practice to get a feel for it... and different density foam behaves differently, so each will present it's own learning curve.

For body work, you'll want a dense, small grain foam for the smoothest surface... I use pink & blue insulation foam for practice, one-offs & non-critical layups... you'll want to source a provider for larger 1 piece tooling foam blocks. Stay away from floatation foam as it's very porous.