Choose your hair dye. Good Housekeeping has a list of hair dyes they've tested against grays that stand the test well. (See Resources)
Read the instructions on your box. Not all dyes are the same when it comes to directions. So you don't want a botched job, read up!
Put on your old shirt and your rubber gloves. Mix the dye, if necessary, according to the instructions. You're done with the prep and ready to move on to the dirty work.
Pull your hair back with a hair elastic and apply the dye to your hairline, all the way around your head. Remember, you're concentrating on your roots here, but make sure the hairline is saturated at least one or two inches back into your hair.
Take your hair down and part it in the center with your comb. Apply dye down the center of the part and then use your fingers to make sure it saturates into the first couple of inches of hair on either side of the part.
Make another part, approximately two inches to the right of your center part. Apply dye as in step five. Now keep on makin' those parts all the way down the side of your head. When you're done, do the right side in the same way.
Run your fingers from your roots through to the bottom of your hair to disperse the dye through the rest of it, to help it blend. Don't dye the rest of your head. We're just talking about covering your roots here. Most dyes dry your hair out, so if all you need is a root touch-up, then fully saturate the roots only.
Leave your dye in for the time period recommended on the directions. This time is typically 20 to 25 minutes. Don't leave your dye in for longer than 30 minutes, the longer the dye is on your head, the more the chemicals can do damage to it.
Hop in the shower and rinse your hair under the shower head until it runs clear, which means you've got all the dye out. Most dye kits instruct that you not use shampoo and instead only condition your hair, with a conditioner that was provided in the box. These instructions are to allow the color to fully permeate your hair. Shampooing may cause a slightly less vibrant color right after you dye, but isn't going to do you any harm either. Really, that's up to you.
Follow up with conditioner. This helps to combat the harsh chemicals in dye that affect hair. The conditioner that comes with your kit is great to use, it's extra thick and moisturizing. If your kit didn't come with dye, no worries. Your regular conditioner works too. Conditioner for color treated hair is ideal, but not necessary.
Dry and style your hair as usual.