A few tips:
Go easy on the glue! There should be a very thin film of tacky contact cement on your pieces and then press the edges together and hammer down with a flat, smooth hammer to set.
Leave a little extra leather in your patterns so you can trim the edge after gluing. This allows you to have a perfectly smooth, flush edge cut with a razor blade.
I personally don't sand much if any when I leave extra and cut it with a sharp knife. The edges should be already perfect.