I've only done a couple but this is how I did it. I pre-stretched the leather to fit the grip. These were on selfbows but I make the grips more like a recurve or a locator on a longbow. Once it was stretched and trimmed and I had the holes punched, I put contact cement (Weldwood I believe) on the leather. I didn't put any on the bow handle, just the leather and don't let it tack up. I also didn't put it on all of the leather, just the portion where it would contact the back of the handle in the deepest part of the grip. Kind of down the center of the leather away from the lacing holes.
Then put the leather on the grip. The adhesive will hold it pretty good on the back of the handle in the throat of the grip but leaves the rest of the leather that wraps around free. This allows the leather to stretch while you are stitching it up but the adhesive in the deepest part of the grip keeps it tight while you are shooting and prevents it from slipping.
You can apply a little more contact cement on a little more of the leather if you want, but don't go all the way to the lacing holes. Part way around the grip is plenty. Clear as mud! Haha. I'm sure there are guys on here that have done a bunch more than me that can give you some tips, but this method worked pretty good on the ones I did.
Aaron