You can either use a rotary tool(Dremel) or cutters if the branch is thin enough.
To attach, either superglue gel or epoxy. Not sure if there are other methods, but when attaching frags, I apply the superglue onto the rock with both pieces out of the water and the surfaces briefly wiped dry. I then wait for the superglue to thicken up drying for about 30 seconds then place the entire piece back into the water. The whole process takes less than 5 minutes and can be done also with epoxy, although you can skip the drying stage. If taking the rock out isn't an option, you can apply the epoxy right in the water and mold the frag onto the rock. Not sure if it's necessary but I'd recommend mixing the epoxy out of water before applying. If you're using a skimmer, shut it off for a day or so after applying epoxy, your skimmer won't react well to it.
Hope this helps ^_^