It's really a matter of splitting up the geometry so that separate fully intersecting surfaces are produced that are good candidates for Trimming with one another... it can and does get quite messy on screen, but is doable.

On the other hand, if all you really want to do is produce V-Carved shapes, then I would strongly suggest you buy CamBam and use the totally free and unrestricted V-Engrave plugin - it works a treat and CamBam is the most bang per buck of CAM software anywhere, anyway.

See here: http://www.cambam.co.uk/forum/index.php?topic=3513.0

 

Martin.