Base your hair extremely well.
Save the edges and the back of your hair (kitchen area) for last.
Slather something over the previously relaxed hair to protect it. If you buy the Affirm no-lye, it comes with Protecto, a gel specifically designed for this purpose. If you don't use Affirm, try something else. I used my old petroleum/mineral oil based hair cremes that I ditched since joining this board to protect the previously relaxed hair.
Ideally, get a friend to help you. Alternatively, use mirrors so that you can see what you're doing to avoid overlapping.
Aim to smooth the hair evenly; if you don't, you'll have uneven roots throughout your hair.
I agree with Pebs that you should use a neutralizer immediately after you relax, but depending on the system you use you might have to relax, condition, then neutralize, in that order. You'll see this in the instructions for both Affirm and Motions systems.
Make sure you completely wash the relaxer out of your hair or you'll experience breakage.
Stick to the timing instructions on the relaxer & rinse when the max time is up, no matter where you are in the process. Leaving relaxer in for too long is a sure-fire recipe for breakage.
Be extremely gentle with your the wet, newly relaxed hair.
