Step1
Unless you've already got a buzz cut, you're going to need to cut your hair as close to your scalp as possible. Clean and dry your hair if necessary. Then, with hair clippers, cut your hair as short as possible. (Do so in installments if you're having trouble, or if you're not sure you want to take the look all the way - start out with the clipper's 1/2" setting, then 1/4", then 1/8", then unguarded.) You could do this with scissors, but it's a painstaking process and it's easy to cut yourself or someone else if you don't know what you're doing. Work on a dry floor so the area can be swept easily. Get as much off as you can; the less hair on your head, the easier it will be to keep the razor clean - and the cleaner your razor, the more control you'll have over it and the less strokes you'll need to finish your head.
Step2
Either in the shower or at the sink, wash your scalp, using either your favorite shampoo or a facial cleanser. Rinse with hot water and leave area wet.
Step3
Lather up your scalp using whatever shaving product you're comfortable using (gel, cream, oil, whatever.) Make sure to work the product into your scalp; the better lubricated you are, the better protected you are.
Step4
Using smooth, short strokes and cleaning your razor frequently, start shaving your head from front to back. I usually start at the top, work my way down to each side and finish with the back. If this is your first time, you should know that the shorter you've cut your hair, the easier the shaving will be. (If you've got more than a couple days' worth of stubble, the razor will have to be cleaned CONSTANTLY.) Move slowly. For the back of your head, use the fingertips of your free hand as your "eyes" to feel out the contours of the back of your head, then stroke slowly. Don't force the blade along; as long as your razor is clean and sharp, you shouldn't have to. Make sure to get behind your ears and your neck. If you accidentally nick yourself, don't panic; fold some cold, wet paper towels up ahead of time, and use them if necessary to compress the cut until it stops bleeding.
Step5
Once you can't feel any more hair (a stubbly feel is OK, so long as you can tell the hair's gone), rinse your head with cold water and pat dry. Make sure your razor is clean and dry before you store it. Apply a mild aftershave to soothe any damage and protect the skin.
Step6
Maintenance; shave as often as you'd like (every other day at the most). I usually stick to two or three times a week. Realize that the longer your hair gets between shavings, the harder shaving will be. If you go more than a week or so, you might have to go back to the clippers to take your hair down to a buzz cut before you shave. Wash your scalp using either a dab of your old shampoo or a facial cleanser; either will be good at washing away the oils that will now build up on your scalp just as they do on your face. Also, very important; use sun block, and try to get out in the sun for a little bit at a time every day until the top of your head is as at least as tan as your face. Make sure to wear a hat whenever it's cold; your head won't feel too much of a chill, but it will be losing a fair amount of heat.