You can mix bleach to your desired concentration -there's no set rule. I'd dilute more to start with and add more if you don't see the changes you want. I dab a wet Q-tip with the powder and put about 150mL in it to start. Andrew's method works, but I've found it easier to just get some gloves and drape the print out of the fixer onto your arm, making the part of the print you want flat. I blow off some of the fixer to allow the bleach to set in better. If I need resistance, I leave the fixer on as the fixer kills/slows down the bleaching process. When I see the area highlights/midtones are turning, I flip the print back into the fixer, pull it out and see if it needs more. I can do this as many as 10 times before I hit the contrast I want. There's more room for error in mishandling and creasing the print with this process, but I find water acts too slowly to stop the bleaching process and can leave a running trace of reduced density if it sits too long or if the concentration is too strong. I also avoid putting the print on a sheet, because if you see it changing and need to act quickly, you can't stop it in time. If you use andrews method, I'd get a cotton ball with fixer ready on hand and use that first to kill the process quicker, then rinse.
Bleaching is an art and you need to just experiment and see what works. Use some workprints or test strips to practice before working on the actual print. I've gone back to prints that I bleached when I learned about the technique and now scoff at them because my bleaching technique was not very refined when I started. Keep in mind that you should never need to bleach too much -it is retouching that just adds a bit of punch to the photograph. If you are bleaching to make the print lighter, try adjusting your exposure first. You shouldn't use bleach to compensate for exposure, but instead use it only to control local contrast. Also, let the print dry down before you bleach so that you can accurately gauge how much bleaching is necessary, if any at all. Depending on the size of the area, I use a cotton ball, Q-tip or a small brush.