Most Helpful Girl
Well since you're going with one color it should be easy and, if you're going darker, even easier. If you want to go lighter things get tricky because, for the most part, you can't just dye your hair a lighter color, there needs to be some sort of bleach in the mix, or bleaching step to do so. In which case it's not so much dying as it is bleaching. Think of a shirt- when you put tie dye on it it changes color, but with enough washes it can fade. But what happens when you spill bleach on that same shirt? Color's gone forever, that sh*t doesn't wash out. It's the same with hair. So, unless you want lighter hair until new hair grows in, stick with just going darker.
Now how to make it look good-
1. Don't put product (of any kind) in your hair today. Feel free to wash it with shampoo (try not to use conditioner) but don't use gel or spray or any of that. Your hair absorbs those things (same with conditioner) and leave less room for it to saturate itself with the dye, so you'll get a less intense color, not as long lasting and potentially uneven.
2. Wear a ratty T-shirt and drape a ratty towel over your shoulders that way you won't get messy.
3. When you go get the dye try and buy a brush and little container, the squeezey bottles are horrible, I hate them and you'll get a much nicer and more even finish from a brush. Go to any beauty shop (Sally's is the big one in my area) and you'll see.
4. Start on one side of your head, flip all the hair over to the other and go through, doing the underside then the topside of the roots of each layer of hair. Once you've gone over the whole head (including the back) go through layer by layer again and do the rest of the hair. This will ensure an even tone and that you don't have weird, miscolored roots.
5. During the waiting period where the color sets, put a blowdryer on low heat and low power and just go over your head for at least part of the time, it will help the color set better.
6. When you wash it out use a special color strengthening shampoo and conditioner.
7. Don't be afraid that even a day or two later color may still be draining out, it happens, with normal colors it'll last at least two or three months and that excess color that's draining out shouldn't because you to worry.