Hold one of your palms out flat, as if you're going to give someone a high five.
Wrap a cloth tape measure around the palm of your hand, at the knuckles just below your fingers. Record the number in inches. Consult a size chart for the brand of glove which you are buying and select the corresponding size.
Find the type of football glove made for your position. There are gloves made for individuals who touch the ball and others who don't, such as linemen. The linemen's gloves have more padding at the knuckles than other gloves.
Try on the glove size that matches your measurement. You may find your fingers either aren't long enough for the glove or are too long.
Try out one glove size smaller and one glove size bigger to see if you like the fit better. Make sure you can make a fist while wearing the glove; if you can't, the glove is too tight.