Made these from unraveled yarn taken from two old, worn-out pairs of mittens, one I got for free, probably started out as military surplus, and one I bought second-hand for about five euro. Both pairs were worn thin and had some semi-well patched holes. I attached all the worthwhile pieces of yarn together, and doubled it (one strand from each old pair). For the thumbs, I ran out of yarn, but felt it would go against the spirit of the project to use new yarn, so I went and found another abandoned single mitten with a large hole in it to finish.
The technique is very simplified crochet, as it's the only yarn-type thing I know how to do. These are also the first ever proper woolen garment I've finished, although I have done a fair bit of repairs. I'm rather happy with how they turned out, except that the thumbs are a bit lumpy and clumsy. Even so, they're very warm and mobile enough to do all the things I want to be able to do with mittens (carry things, ski, use an axe...) It works. Just looks a bit strange. If it ends up annoying me too much I might redo the thumbs at some point.
I did take pictures of the starting pairs and some stages in the process, but unfortunately I think they are on my partner's old phone, which is now semi-dead, so they may be lost. If they reappear, I'll post them later.