I did try to make other mittens on long loom and you might look at my other topic on knit topic called thumbless mitten. This one it is not and I am just showing you to part of crochet on thumb part how to attach to it.
This picture from long loom knit to create a hole to create thumb part to crochet it:
This picture to crochet how to attach to it:
This is just to start crochet from attach it:
This picture already create almost there to thumb closed up:
This picture already closed it up the thumb part: