I recently found my old 'Classic Mitten' knitting pattern that was given to me over 30 years ago by my grandmother. I've made it many times when my children were little.
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.