There are several ways to get collars on deer but I guess squirrel covered it all in his unbiased and open description. You can capture deer with drive nets which are ground nets, the deer are pushed into with a helicopter. You can use clover traps, which are basically a baited live trap for a deer. You can capture deer with a drop net basically a huge circus tent sized net which is dropped with a remote button on the deer. Finally there are net guns from helicopters, which are shot using a .308 charge. The deer have several things done to them. Body measurements are taken (chest girth, hind foot), tooth aging, blood is drawn, a body condition score is taken, and a few odds and ends. Most studies done are performed on does, not bucks. I have personally seen quite a few with collars. I had one old doe I used to watch every year.