I was scared to do this to my turkeys, but once I did it 3 times to 30 turkeys, it got easier!
Basically, I had to first catch the turkey, then someone else held it securely while I stretched out the wing out so I could see the "flight feathers," like in the link above. Then I trimmed with a sharp scissors just the longest feathers. You can kind of tell where to stop because there's the really long feathers, then kind of a line where a second row of shorter feathers is -- don't cut there! Anyways, it's not hard. Twice I cut too far, and the quill part of one feather bled which I felt bad about, but I got a good sense of how far up to trim. The feathers just grow back in a month or more, so I had to do it again.
Just stretch the birds wing out, and you'll see. Dont' cut the actual flesh and bone of the wing, and don't cut the heavily feathered area. Just cut part of the very large "flight" feathers off.
Hope this makes sense. I printed out the diagram from the link above and set that on the ground when I did my first one, just to make sure I was doing it right.