https://www.stanwinstonschool.com/videos# At the time I post this they have 50% off sale. Enjoy. You can preview them to see if they have what you want.
I recommend:
To create the life cast:
How to Life Cast
For Sculpting:
Character Design I and II
Sculpture Techniques
To bridge sculpting and mask making:
Character Makeup Sculpting
To create the mold for casting:
Making and Ultracal 30 Stone Mold
Now, these don't cover making a mask explicitly but they all have information that will be helpful. I'm getting Making a Monster Suit during this sale as I think there may be a fair bit of actual casting in this video.
How to Make a Mask I & II may help you, but he makes a mask that gets strapped on, which doesn't sound like what you want.
Monster Makers has a great little e-book for sculpting and casting. They also offer latex for making masks. Their latex is what I'm going to get for my first mask. They have some instructional videos, but I can't speak to their content.
Ignore the music (if you can). He goes over slush casting with latex and silicone. I'd been lead to believe you couldn't slush cast with silicone, which seemed odd to me because there are brushable silicones. You have to be careful about what silicone you get. I understand tin-cure silicones can be toxic to humans. Latex shrinks (general rule of thumb seems to be 5-10%) so you have to sculpt that in.
This is another good video of slush casting with latex. Alex the Movie Geek has a
pretty good video for making a latex Batman Cowl on the cheap.
Silicone is more expensive than latex, which is why I'm going to make mine with Latex first. That way I can make mistakes with the cheaper material, then make new mistakes with the more expensive one. As someone who was (and still is) in your boat I think someone who's done this for years should sticky some stuff like this. I've probably provided you with some bad information, so make sure you research and verify everything I've written. Peace and good luck.