Some points that help me:

• Leather needs to be flat. I use masking tape to hold edges and help avoid the laser guard catching the leather (if the edge curls up). You can also wet down leather with a damp cloth, this will help folds in the leather come out.
• I process in following order: engrave (green), cut inner T shaped hole (blue), and cut outer profile (red) last.
• I use a little air assist.. I find too much air causes the leather to burn more and char a lot..
• I apply beeswax leather protector after laser processing to help waterproof and protect the leather.