Part your dry mane into four sections: two in the front and two in the back. You don't want to fight your hair while trying to do a precise application, so keep it in its place securing it with plastic clips on each section.
Protect the skin of your precious face from accidental (and painful!) relaxer burns by smoothing a little petroleum jelly around your hairline in the front and back and on the tops of your ears.
Stand in front of a mirror and check out the two textures: straight at the ends from previous relaxers and the naturally curly texture closer to your scalp. Identifying how much new hair growth, or naturally textured hair, you have is majorly important, and could mean the difference between total hair breakage fiasco and not.
Remove one of the plastic clips from the back part of your head, set it aside and grab your rat-tail (horrible name) comb. Start closest to your nape and use the tail of your comb to create a ¼-inch horizontal part.
Coat the previously relaxed portion of the section with conditioner. This will create a barrier, much like the jelly on your hairline, that the chemical won't be able to cross. Your relaxed tresses will be safe from overprocessing.
Glove up using a pair of thin latex or plastic gloves (included in your relaxer kit). Mix your relaxer according to the package instructions, then set a timer for the recommended processing time for your hair type.
Dip the applicator brush into your prepared relaxer and paint it onto the naturally textured hair, starting 1/4 inch from the scalp. Stop your application right at the line of demarcation (where your natural textured hair ends and your previously relaxed, conditioner-slathered hair begins), and don't cross it, no matter what. Lift the parted section and paint the underside with relaxer, but remember -- only the new growth.
Continue making parts and carefully applying the relaxer to your new growth until that section's all done. Don't stop there; you've got your whole head to go, so repeat the process until you've got all your new hair growth covered in relaxer. But work fast so that you don't exceed the time allotted.
Go back to the section in the back of your head where you first applied the relaxer. Smooth the relaxer that's on your new growth, using your hand (you're still wearing those gloves, right?) or the back of your handy rat-tail comb. This isn't the time to get carried away, so don't start spreading that relaxer beyond its boundaries. Only smooth it where it's been applied, on the new growth. Continue until you've smoothed each of the sections being retouched.
When time is up, rinse out the relaxer completely with warm water until all visible traces have gone bye-bye. Deactivate the relaxer by shampooing your hair two to three times (or until you're satisfied that all the relaxer is gone) with the neutralizing shampoo that came with your relaxer kit. Follow with the conditioner that accompanied the kit.
It's difficult being accurate when you're struggling to see the new growth versus previously relaxed hair at the back of your head. Ask a friend who owes you a favor to help out with applying the relaxer in that area.
Always condition your hair following your relaxer retouches.