For a harder leather that you can shape you need vegitan. It comes in several "weights" 4-5oz. will likely be good for a mask, but if there's going to any kind of stress on it you'll need to reinforce it or use a heavier weight (I use 7-8oz., the next size up, for collars). Molding tutorials can be found:
http://www.cosplay.com/showthread.php?t=274695http://www.instructables.com/id/Sculpted-Leather-Mask-Tutorial/
http://www.armrd.com/wet-moulding-leather/The bottom "soft red" you refer to... did you want that the be shiny or more of a sued? You can connect it by using barge cement (the kind with toluene, not the other kind- it sucks), rivets (You can likely use Tandy's Rapid Rivets here. They look better and are easy to use, but aren't heavy duty so don't go making any saddles out of them

), or sewing or any combination there of. If I was making the mask I'd glue it and then sew it to reinforce it, but I over engineer things.
Leather is really best sewn by hand. Two main types of stitches are used: the lock stitch and the saddle stitch. A lock stitch is what your sewing machine uses, and Tandy Leather sells a hand tool that will do the same thing
https://www.tandyleather.com/en/product/sewing-awl-kit. A stronger (and cheaper!) method of sewing is the saddle stitch.
http://makesupply-leather.com/leathercraft-tutorials/how-to-saddle-stitch-leather/https://www.marthastewart.com/1125947/how-sew-leather-handThe book noted in both of these, Al Stohlman's "The Art of Hand Sewing Leather" is worth every penny. If you plan on doing any amount of leather sewing you won't regret buying the book. It's $20 on their website, but knowing what you are doing saves a lot of pain and "do overs".
Riveting and gluing are pretty straight forward. If you are still having trouble finding info, I would check out
http://leatherworker.net/forum/. There is a huge wealth of knowledge and helpful professionals there.
I hope this helps.