When I write a recipe that includes nutmeg, I try to specify that it should be freshly ground nutmeg. I notice that more and more recipes are written this way, where years ago most just said simply “nutmeg.” This is because nutmeg has a delicate, spicy flavor that starts to dissipate shortly after being ground and you really can get a lot more flavor out of a much smaller amount than you would with preground. There is no exact standard of conversion between fresh and preground, but to give you an idea, you might only need 1/4 tsp of freshly ground nutmeg where you might need a whole teaspoon of preground – and you’ll probably get more flavor from that 1/4-tsp.
Whole nutmeg seeds, which are the dried seeds of a nutmeg tree, are sold in most grocery and specialty baking stores. One jar will last you a long time, and the whole nutmegs will retain their flavor during storage. To grate or grind it yourself, you can either use a nutmeg grinder or a microplane. If only a small amount of nutmeg is called for in a recipe, a few grinds should be enough to impart some nutmeg flavor. If you must use preground nutmeg, try to replace your jar frequently so that the nutmeg has as much flavor as possible when you go to use it.