Get the jig Dangus linked to.
A couple of tips from fitting them.
- Tape off the rest of the back side of your frame when filing. It's very thin and you can easily put some bad scars in it with your file.
- Wilson's are fit by marking, tapping the safety down, file, repeat. Be gentle with this or you can mess up the top of the beavertail. Sometimes the arm of the grip safety can hit the frame giving you a false sense that you need to file more on the tangs. That's why you see a lot of guns with a great fit on the left tang and a gap on the right.
- Measure the diameter of the thumb safety you intend to use and buy a little gauge pin off Amazon in the same size to use while fitting. It's just nice not to have the TS in the way.