Place the end of a soft cloth measuring tape in the middle of your forehead, and hold it in place with your finger. Don't use metal measuring tape. You want cloth because it will wrap flush around your head, and you'll get a truer measurement. If you don't have a cloth tape measure, don't worry. You can also dig up a long ribbon and use that instead.
Wrap the tape measure or ribbon around your head. The fabric should sit just above your ears and across your occipital bone on the back of your head. The occipital bone is a fancy, technical term for that funny little bump on the back of your head. Basically, just wrap the tape measure around the part of your head where a hat usually sits. Wrap it snugly enough to get an accurate measurement but don't pull extra tight.
Look in the mirror to see where the tape measure meets in the front. If you're not near a mirror, just pinch the tape or ribbon on the spot where the tape measure meets at your forehead and then unwrap it to take a peek. If you're using the ribbon method, you'll have to use a ruler to measure the length of ribbon from the end to the spot you marked.
Make a note of the measurement you took. It'll probably be somewhere between 22 and 23.5 inches since, according to Kevin Cowan at the Knoxville News Sentinel, that's the average head measurement for a woman. Yours might be a little bigger or smaller, but either way, go ahead and measure your head one or two more times to make sure you get a good, solid measurement.
Use your head measurement to consult a store's hat size chart or give your measurement to a sales clerk. Keep in mind that some hat brands may run small, while others may run large, so try on a hat first, if possible, to make sure it hugs your noggin in all the right places.