If you want nice leather, get it from your local auto breakers. The backs of leather seats gives you a nice big piece, and the backs are generally thinner ( it's cheaper ) than the front surfaces where your butt goes. Always ask before hacking the back out of a Lexus seat, if it's a good seat they want to sell it complete.
The other good source is leather coats from charity / thrift shops.
I have thinned the edges with sandpaper, obviously on the back.
If you are working with thick leather, 3mm and above, then holding the leather edge gently against a grinding wheel will shape it, and 'seal' the edge with the heat. Obviously this could be considered a huge breach of health and safety, so be bloody careful!