There a couple of good threads about that on the practicalmachinist forum in the gunsmithing section. Search for "closing cam" or just "cam."
Here's one to start with: Receiver locking lugs-construction
There's some really good stuff on that forum once you learn to ignore the losers who only log on to crap on everyone else's parade.
Basically, it's a manually operated shaving tool using a helical cam groove; looks dirt simple to make.
If you're building a target or benchrest rifle, you can shorten the closing cams considerably to regain lug area, stabilize the bolt a bit more, and get more lug area. If you're going to hunt with it you'll probably want to replicate the factory cams, which are there to close the bolt on off-size ammo or a dirty chamber.