I have sewn a lot of face masks over the past few months. Here is a photo tutorial of how I make mine. The original pattern that I used to start was from the Craft Passion website. (Link to free pattern)
However, if you have a mask that you like the fit of, you can use it to make your own pattern, as I have shown here. I show the mask made with long stretchy ties that go behind your neck and head, but you could insert a long elastic as well, or put two shorter elastic loops for the ears. It is a versatile pattern.
This is a mask made with two layers of quilting cotton, with an opening between the two layers to insert a filter of you want, and a slot to insert a nose wire.