If you want to smooth it out, I would use modelling paste. Put it on with a brush, but use your fingers to make sure it gets pushed into all of the crevices, then smooth it with the brush again. When it is dry, you can sand it if you want. You can then paint it with acrylic paints. If you want to adhere things like gems, feathers etc, you can use the paint as an adhesive. When you are done, I would finish it off with a couple coats of an acrylic medium, in either gloss or matte, like Mod Podge of Liquitex gloss medium and varnish.
Naomi in San Francisco
PS There are simple directions on our web page about this process, complete with a few pictures.
http://www.mendels.com/mask.html