Firstly, get a rectangle of the leather. size doesn't matter, go by common sense - larger leather = larger pouch.
Fold the shorter two sides in by about an inch.
Sew along the folded edges.
Have this done for both sides.
Fold it in in half from shorter edge to the other shorter.
Sew along the side, although leave about 1cm from the top so the string can be inserted.
Do this tob oth sides, so now the pouch is sewn up and only the top is open.
Insert string in to the loops made.
Then, it's done. Pull the strings to tighten the pouch.