This is a pictorial to show how to knit the thumb method I used for the Erinwood mitts. This method is not technically called a thumb gusset, is referred to as afterthought thumb because it is completed after the rest of the mitt is knit.
Once you have finished the mitt turn it inside out, the purl bumps are easier to pick up than the knit stitches. Use circular needles for this part, it makes turning the mitt right side out again easier. Then you can switch to double pointed needles for the knitting. Start on the right side and pick up the 5 purl bumps. You can pull the waste yarn out as you pick up each stitch or you wait and pull it out after you picked all the stitches. When you get to the end of the first five pick up one more stitch. That will help close up the holes at each end.
This gives six stitches on your needle. Pull the needle all the way through so you can use the same end to pick the rest of the stitches.
Now repeat the same for the lower row of stitches starting on the left side for a total of 12 stitches.
Once all the stitches have been picked up pull the needle so that all the stitches are on the cable part of the needle.
Take the ends of the needles and stick them through the hole between the rows of stitches. Pull them all the way through turning the mitt right side out.
Now you are ready to knit the thumb. I like to leave a long tail of yarn when starting the thumb because it can be used to close up the holes at each end. You may also knit the first row twisted (by knitting through the back loop) to make a more secure thumb. Enjoy your mitts!