If you find you have to sew leather by hand, or even if you have persuaded someone else to do so, the trick is to get a leather needle. It looks a lot like a fairly tough normal needle but the sharp end of it is square in cross-section rather than round. This makes it much easier to push through the leather. Craft shops etc should sell them.

I suppose you can get something similar for a sewing machine - I certainly wouldn't expect a normal sewing machine needle to cope.

Marje