It is a simple job really, did mine today and it took about an hour. I laid out the slot on the receiver including a vertical centerline. Put punch marks for drilling with a small sharp tipped center punch. I drilled a hole at each end of the slot and a few in between using the tip of a center drill (receiver is thin). I filed out the material between a couple of the holes with a small round Swiss file. Then switched to a small rectangular file. Doesn't take long to file it out and square up the corners nicely.
The slot needs to be about .090" wide for the sling swivel to go in with a thumb press fit.
Once you spread the tabs inside you can go in through the selector hole and drive them down nice and flat.