I started out just kind of doing my own thing with pre-made paper mache clay and stuff I pulled out of my recycling bin. Eventually I graduated to a clay recipe found in this book: https://www.amazon.com/dp/B006IEBQPK/ref=cm_sw_r_cp_apa_mUsoBbFWX656T
And I've started using these mask bases and tape bits of cardboard and crumpled paper to them before adding the clay: https://www.amazon.com/dp/B000XQZB3M?ref=yo_pop_ma_swf
Once dry, I sand them down, paint them with acrylic paint, and seal with a clear coat spray.