Initially, crush the Campden tablet with a glass mortar and pestle and stir the powder directly into the must.
Before racking, crush the tablet and pour the powder into 1/3 cup of HOT water. Use a sanitized bamboo whip or a stainless steel fork to beat the powder into the water. It may take a few minutes, but it will all dissolve if you work at it. This cools down the water quite a bit.
When dissolved, use a wine thief (or siphon tube) to draw off a cup or so of wine and mix this with the sulfited water. This sometimes causes small lumps to form, so whip the mixture a little more and then pour it into the receiving jug or carboy.