Normally, molding a sculpt can be difficult, but this method is somewhat simple and easy. Usually, molds of mask sculptures are done as 2-part molds. For simplicity's sake, I'm doing it all in one piece. It is generally easier to do it all at once instead of in halves, but the method does have its disadvantages as well. I'll point those out along the way.
And, as always, we'll try not to spend too much money. You'll have to make a mess, though, and the process will take some time: a few hours to make and clean the plaster mold, and several hours more for casting the latex.
Here's a video tutorial you can follow, or watch as an introduction to the overall process, including both Part 1 and Part 2. The step-by-step Instructables are more detailed and have more advanced tips and recommendations.