Just a little more info for a little more clarification:
In most knitting patterns, once you've finished the cuff, you start to do the aforementioned increases for the thumb gusset (sometimes also called the gore). If you hold your hand in front of you, the fingers straight out but not held together, you'll see that just above your wrist, the thumb actually begins. It doesn't separate from the rest of the hand for a couple of inches higher up. So for those couple of inches, you're going to gradually make increases (usually increase 2 sts, then a couple of plain rows, then increase a couple more sts, then a couple more plain rows, etc.), so that you're creating a wedge that will stick out from the rest of the hand. Now you've come to the part where the thumb does actually separate from the hand-- so you will put the wegde (gusset/gore) sts on a holder. THEN you will usually cast on a couple of new sts (this just makes everything lie more smoothly, it just works better) and then go back to knitting the rest of the hand. Then at the end, you go back and to the thumb. This is a great, simple knitting pattern which my grandmother had-- I have her tattered copy, but it's now (:woot: ) on line! Although they have reproduced in an altered form and it's harder to read. So just be careful about the numbers for whichever size you're using. Here's the link: http://www.freevintageknitting.com/mittens/615-mittens-pattern.html
One other thing about knitting mittens which 99% of patterns don't tell you-- when you go back to knit the thumb, at the end, they'll tell you to pick up those couple of cast on sts you made. The problem is that the way knitted fabric works, that will leave a little gap. So what you do is pick up-- say they tell you 3, then I'd pick up 5-- and then knit those 2 extra sts together with other sts in the next round. You'll be back to the right amount of sts, but you'll have tightened up that gap.