The first step in creating a latex rubber mask is to create the sculpture first. You should have an armature of a head shape you can make out of a styrofoam head or wire and wrap it in tin foil. For more advanced users or if you want to get into sculpting more you can buy a lifecast armature of a head. You can use an oil-based clay or use a water-based clay. Start by gradually adding clay to the armature and form out your shape of whatever creature or mask you're looking to create. You can use sculpting tools / ceramic tools to help in making fine detail or helping with overall shape of your creation.
Once your clay is ready and finished, it is ready to be molded. To make a latex casting, you will need to make a plaster mold first. The mold will be made as a negative of your sculpture you just made. The plaster you use could have the same used for potters and it is called pottery plaster. Start by getting your sculpture ready for the molding process. You may build a wall around your sculpture so the plaster does not fall down on the table when you're trying to put it on. Mix up the plaster according to the specs on the plaster you buy (should say on the bag). Lightly brush or use your hands to cover the sculpture in your mixed plaster. You can do it in layers but do not ever let the plaster dry out in between coats. You should build up enough coats to be about and inch in thickness all around your sculpture. After your plaster mold is set up and hard, you're ready for the next stage.
Remove the plaster from the clay and take out all remnants of clay. If there is residual of clay left, you can wash out with water. Let the mold dry for a day or so until most of the moisture is gone. Now you're ready to create the mask. You will need to find liquid latex rubber to cast your mask. You can find this online just search for latex mask rubber. Pour the liquid latex rubber into your mold. Let it sit for anywhere from 20 minutes to and hour depending on how thick you would like the mask to be. Pour your latex back into your container for later use. Let the mask fully dry in the sun and when its dry you can pull it out.
The next step would be to cut the eye and mouth holes for ventilation. You can use a regular scissors for this. Now your mask is ready to paint. You can use any craft type paint, but you will need to add 1/2 of latex rubber to the paint in order for the paint to be flexible. Once your creation is painted you're ready to wear your mask.