Hinges can be made out of many different materials. I'm using 2.0 mm thick aluminium for mine, it is light and very durable.

1 + 2)
Most hardware stores have aluminium sheets available. Best use a 2.0 mm thick sheet, 1.5 mm might work as well but I prefer it to be more stable. I created simple patterns for the hinges out of cardboard which I use to sketch the shape of the hinges onto the aluminium sheet.

3 + 4)
I use a band saw to cut the hinges from the aluminium shape, so the edge of the aluminium is pretty rough. I just sand it till is nice and smooth.

5)
Next I mark where the hinges are going to be attached on the mask. The hinges will be glued onto the resin base, then I will drill holes for screws to get some extra durability. I have seen quite a few masks where the hinges came loose because they were just glued.

6 + 7)
I always scratch the surfaces before gluing anything, otherwise it might not stick at all.

8)
This image shows the installed hinges from the outside and inside. The red circles mark the areas for small grooves I always forget. These grooves are used for the rubber band which pulls the lower jaw up. The image 9) shows a hinge with the grooves added as well as a rubber band.

The reason why I use a rubber band instead of a spring is just simplicity. I got lots of rubber bands in my kitchen, but no springs. Further the rubber band can be removed easily. If it is too weak, I just wrap it around multiple times till its right. So far none has torn for me.

10)
This image just shows the installed hinge from the front. I always install them so that they point a little bit to the outside, otherwise the would collide with my ears.