It's not difficult if you understand how the thumb safety blocks the sear.
The thumb safety will come a bit oversized in that area and if you try to install it in it's "safe" position with the grip safety off you can see where the sear will block it from entering the frame. CAREFULLY file that portion away until it BARELY clears the sear. This should take you some time.
The, smooth out the rough edges on the thumb safety where it rides against the frame as some can gall.
