Sorta hard to summarize, you kinda need to know enough about blender from the get go.
The best tip I can give, as long as you understand the basics of blender, is to use a modifier layout that looks something like
And build out the mask using flat planes.
When I create masks I do it by creating bands or strips of material and laying them over or connecting them to one another, sort of like sculpting with clay overtop something.
I also always have a model of the head I’m building it around in the program so I can be certain it’ll fit and line up properly. If you don’t have a model of the head you’re designing for it could become a real challenge.
If you’re trying to make an inika style mask, you could try using sculpting mode once you have a solid base to work off of.