There are many ways to chemically straighten hair, including pricey keratin treatments and Japanese hair straightening. Perm solutions are another method, and can be used to give you something called a "straight perm." Getting any treatment done in a salon is the best thing you can do for your hair, but you can give yourself a straight perm at home with perm solution and a wide-toothed comb.
Before even considering attempting this kind of straightening, you have to make sure you're a good candidate. Perm solutions are permanent. If your hair is naturally wavy or curly with no damage and good elasticity, you could do a straight perm at home. Virgin hair is the best for the process. If your hair has been dyed, colored or bleached, you are at major risk for damaging your hair. Double processing can cause your hair to fall out, and you must seek professional help if that's a risk. That said, some perm solutions are formulated for hair that's been colored.
A pro will be able to assess the health of your hair and determine how to proceed. If you do damage your hair, or even just hate the look, you're just going to have to wait for it to grow out—and keep in mind any new growth will be your natural hair texture. Your hair will be straight-ish below, and wavy or curly at the roots. If you're going to give yourself a straight perm, you have to be OK with that. Still, typical 2A-4D style curls are best straightened in a salon, by a professional who has experience working with your hair type.
Regardless, if you know your way around chemicals and solutions and are confident with your hair, trying to relax your hair at home is feasible as long as you do it right. If you've tried to color your hair before and messed it up royally, go to a salon. If that's not possible, get a friend who knows what they're doing to help. You can buy well-reputed home perm solutions, such as Shiseido's, online, but make sure you're purchasing the best solution for your hair type.
In order to prepare for a straight perm, wait at least two days after your last shampoo to apply the perm solution. This is because your hair will be coated in its natural oils, protecting it from damage. It's imperative that you read the directions that come with your kit thoroughly. You're giving yourself a perm without the rollers, so the times on the box are the times you should stick to. You risk damaging your hair otherwise. You'll need an old towel, clothes you don't mind messing up, Vaseline, a wide-toothed comb, gloves (plastic or rubber), a perm kit, flat clips (optional,) and shampoo and conditioner.
Consider roping a friend into helping, too—four hands are better than two.
Perm solution works best on lightly damp hair, so rinse your hair so thoroughly in lukewarm water that every strand is coated, then towel-dry. Don't use shampoo or conditioner unless one was included with the perm kit. After rinsing, brush or comb through your hair several times to distribute your scalp's natural oils and make sure there are no knots or tangles.
Next, apply a thin coat of Vaseline along your hairline and on your ears to keep the chemicals from burning your skin. Make sure you have on an old t-shirt or sweatshirt that you won't mind messing up with chemicals. Drape an old towel over your shirt so the chemicals don't seep through your shirt onto your skin. Put on gloves, and section your hair into four sections by parting straight down the middle, then across lengthwise. Keep each section separate by wrapping it in a loose bun. Work from left to right, starting with your first section, applying the perm solution as close to your roots as possible and then combing it through to the ends.
If your hair is particularly thick, you might want to use your fingers to work the solution through, as well as the comb.
If you have a friend helping you, have them use flat clips to weigh down your hair to straighten it as much as possible. To do this, wrap a piece of paper (it'll come with the kit) over a 1 1/2-inch section of hair and clip the end so it weighs hair down. If you're working alone, you probably won't be able to clip your hair yourself. Instead, comb the solution through the hair at least 5 times before moving on to the next section. Work your way through all four, but remember to set a separate timer for each section, or you risk serious damage.
If you're working alone, comb through your hair for the suggested amount of time—probably about 15 minutes—per section. Always follow the packaged instructions.
Next, you'll want to rinse the perming solution from your hair and apply neutralizer. If you had clips in, take them out, and make sure to wash thoroughly (but don't use shampoo unless the box tells you to.) If your kit comes with a neutralizer, apply it the same way you did the perming solution. You can put the clips back in place if you used them, but you can also just leave them off and comb through your hair a few times so it stays straight. Leave the neutralizer in for the recommended time, and then step right into the shower and rinse out the chemicals, making sure not to squeeze hair or bunch it up.
This is where you shampoo and condition. After you get out, dry your hair by gently patting it with a towel. To protect hair from kinking or from creating strange bends, avoid pulling on, washing, or pulling hair back for at least 48 hours. The first 24 hours are critical—you need to make sure you disturb your hair as little as possible.