there is a screw up inside the grip. You just undo the screw, take the grip off, don't lose the selector spring, put the selector spring in the new grip, slide it on while lining up the selector spring and making sure it doesn't bind, and screw in the new grip. simple.
Oh, and when you pull the old grip off, you might want to cover the pin for the selector with your finger so it doesn't fall out.
Just so you know, I did it and I had no idea about any of the selector parts that needed the grip to stay in place. Basically, the whole mechanism fell apart and I had to put it back together. I figured it out, though, and it didn't take long, and I'm not that smart... so I'm sure you can do it.