Here are a few tips that can help when constructing masks, without for the moment getting into specific materials and techniques:
The most desirable qualities for a mask are that it blend organically with the actor's head and/or body; that it is alive and appears to change expression; and that it has a directional quality or focus.
To present a meaningful level of transformation, a mask should be either larger than the human face, or smaller, as in the Japanese Noh.
You should think of a mask as a bowl, and not as a plate. There has to be depth to contain the human face. The distance front-to-back from the tip of the nose to the ear is about as great as the width of the face. Building a mask over a casting of one's own face helps to make this clear. As you are modelling, look as much from the side as from the front. If possible, don't work with the mask lying horizontal on a table, but place it in the vertical position as it is intended to be used.
Working on a table top often leads to another problem, especially if the sides of the mask extend down to the table. The flat plane of the table passes under, and seems to slice through, the back of the mask. When the mask is worn, seen from the side, there is a straight vertical line at the back of the mask. The mask then does not blend organically with the actor's head, as it would if the back edge were curved. If you place a well-made mask face-up on a table top, you can rock it up and down.
It is the sculptural qualities of a mask that make it come alive and appear to change expression when being played. It has less to do with the way in which it is painted. In fact, I have seen many a good mask destroyed by over-painting.
That said, it is possible to paint masks in such a way as to enhance the sculptural qualities. One can apply highlights, or allow the sculpture to assert itself by spray painting from a single direction, or by flicking paint at it from the bristles of a toothbrush. One can take advantage of certain sculptural planes that are only visible from certain directions, for instance around the eyes, by painting them in distinctive colours that will appear and disappear as the mask is played. "War-paint," and the purely decorative painting so typical of carnival and Mardi Gras, should be avoided if the mask is to be playable.
Lighting is extemely important. Good contrast of light and shadow, or cool and warm, on mimimal painting does the most to emphasise the sculptural and expressive qualities.
Interestingly, masks made by amateurs are often very alive and playable because the maker's absence of technical skill results in masks that lack uniformity. In other words, because the character and expression vary according to the viewing angle, they become alive when moved.
If a mask is to be able to tell anything that resembles a story, it has to be visually clear to the audience where, in the mask's surroundings, its interest lies. It has to have a clear, directional line of focus in order to be able to articulate its relationship to the universe. Typically this would be the eyes or nose, but any kind of prominent sense organ, real or invented, will serve. It could equally be the oval disk of a flat face that gives it a clear directional quality. Without this quality the mask will lack intelligence and the ability to communicate.
And then of course the actor needs to know how to exploit the opportunities presented by a well-made mask!