I use pop studs from Tandy (Bought through the Identity Store) and these need a setting tool which has to be bought seperately in order to do a really good job of fixing them.
If you want to do more sheaths and leather work it's worth investing a couple of quid in the right kit to do it well, but if this is a one-off project then try one of the bigger department stores like John Lewis and look in the Haberdashery Dept. They sell little sets of pop studs with a setting tool included.
Fixing the keeper strap to the sheath can be done with either a leather rivet or by stitching it to the sheath.
I'd recommend that you fix the strap to the sheath with one half (The bottom bit) of the pop stud fixed to the end
before fixing the second half of the pop stud ("top" half) to the other end of the strap. This way you can be certain that you'll be fixing the stud in the right place to make it a decent fit to the knife handle as it's easy to make the strap too small as stitching or rivetting it to the sheath will take upa little of the available length and can lead to it coming up too short if you simply measure around the handle, fix the pop stud then fix the keeper strap to the sheath.