If your talking about prevulcanized latex mask . low down and derby like the ones we had as kids . its easy .
Sculpt in wet clay as detailed as you like but think about undercuts that you don't need. You can paint soap on your sculpt if you've let the clay dry out a bit. Then its a straight plaster one peace mould. make the wall thickness minimum 1 inch and try and keep it even all over.
When the plaster has gone off pull out as much of the clay as you can. You can wash out the mould with garden hose-shower . because you are going to put the mould in the over on a low heat for a few hours to get as much moisture out of the plaster as possible.
Then it comes to the prevulc latex . you can use water based paint to give this a base colour. ( otherwise you get that condom yellow ) this is where the experimenting comes in. Fill the mould with the liquid latex and leave it for 5 minutes and pour it out. this is what give you you wall thickness. Your basically slip moulding the same way cups are made.
Depending on how dry your mould is, the plaster wall thickness, the time you leave it in, is what determines how the moisture is sucked out of the latex giving you its thickness.
ass for paint you can use marla balloon paints but be warrens this make the latex expand before contracting back, so the a period when you think its all gone wrong.
good luck.