It depends on the strength you require.
A double fishermans will work but you won't be able to load it because it will pull back together until the two knots lock.
A prusik knot or blakes hitch will lock under load and is adjustable.
I usually tack the upper left corner down and strip the leather from the right side.
Make sure you have a new or very sharp blade.
Sometimes it's necessary to cut the leather into smaller manageable pieces.
I usually wet it and then bake it in the oven but there's a fine line between hardening and ruining.
Lowest temperature for twenty to thirty minutes max.