A glass muller is used when making paint by hand, to grind pigment and to coat it in medium. The base of the muller has a very slight roughness or fine tooth to it (the glass looks "frosted"). The base is usually slightly convex to encourage the pigment to "squeeze out" between the muller and the surface you're grinding on (usually glass too). Mullers come in different sizes; choose one that's comfortable in your hand.
You grind pigment to make the particles finer if needed or desired. Knowing the optimum size to get the best color from different pigments is a rare skill these days. Most painters buy pigment as a jar of "powder" rather than in lumps, and use a muller only to mix the pigment with binder to make paint. What the binder is depends on what type of paint you're making, for instance oil for oil paint, egg yolk for traditional egg tempera.
To use a glass muller, hold it with one hand around the handle and the other on top, then move in a gentle circulation motion over the pigment/medium. The aim isn't to apply pressure, but rather to moving the muller around, to get all the pigment evenly covered with medium.
A new glass muller isn't cheap, being such a specialty item. As a substitute, look in junk shops for a sturdy glass stopper with a flat top from an old jar or bottle. It won't have a rounded handle to fit comfortable in your hand though.
