1. draw around your hand fairly tightly. Don't spread your fingers too wide - just enough to get the pencil around them. Make sure you mark accurately where your "webs" are. Stick your thumb out a little futher to mark the webbing between the palm and the thumb. Also make sure to mark where your palm starts - the top wrist wrinkle is a good point to transfer.
2. measure around your hand just below the knuckles. Compare to the width of your handprint. My circumference is 7" and the handprint is 3.25" wide - the difference being the thickness of my hand (0.25") . I'm guessing that this is pretty average for most people (ex. my husband!).
3. draw lines on your handprint to get measurements (see photo). The finger length measurements are from the tip of the finger to the "webbing". The thumb width is going to determine the gusset size.