It is a common pattern and can be found here:
http://freevintageknitting.com/mittens/615-mittens-pattern.html
I never liked the way the pattern increases for the thumb gusset so I think I found a better way.
The update I did to my pattern is this, on the thumb gusset instead of "increase in the first stitch", I changed it to M1R, then after "knit 1, increase stitch in the next stitch", I changed it to M1L. This makes the increases look nicer.
Here is a link to a Youtube video that will instruct you how to M1R and M1L if you need it.
http://www.youtube.com/watch?v=gGbOiIvWLjc&list=FLcBzJHP_L1FiH0Vlhw9jdJg
I also had problems when I reattached the yarn for the thumb. I would have a small hole. I found another Youtube video that finally, after all these years showed me the correct way.
http://www.youtube.com/watch?v=OQPiqp4KGGo&list=FLcBzJHP_L1FiH0Vlhw9jdJg
I made two small pair of mittens and never had any trouble after making the above changes.