Use your fingers for a pattern. Lay a piece of paper toward the edge of a table. Lay your fingers down on top - index finger seperated about 3/8" and ring/middle fingers together, with the middle finger bent somewhat to make the fingertips fairly equal. Then trace around them, leaving about 1/8" margin. As far as how long to make it, for the type with the finger hole, you'll need to add enough length to make a big enough hole for your finger to go through. If you use construction paper, or some other kind of heavy paper, you can make trial tabs until you get everything just the way you want. Then transfer that one onto your leather.

Just make it look kinda like this:

You say it's fairly heavy leather. If it's thick, stiff leather, odds are it won't make a very good tab. Stiff leather is real bad about taking a groove, hanging up on the string and not giving a slick release. Sometimes you can soak a piece of stiff leather in neatsfoot oil and work it over real good, and make it a lot more flexible.

But then, a jug of neatsfoot oil will cost about as much as a tab...[&:]