The No. 1 rule that "professional" stylists and people in general ignore is the need to do a strand test. It is out of lack of knowledge and mostly the rush that people get into doing a relaxer on their entire head, often using a relaxer formula that they have never used before! A strand test is: taking a couple of strands of hair and testing your application processing. You apply the relaxer as you would for your entire time and do an estimated time test and smoothing test (amount of physical manipulation with fingers or back of comb to undo the curls so that the relaxer can adopt that structure). This strand test will let you know whether your timing is correct or should be adjusted. I do five strand tests when I relax my natural three year hair. In addition, companies change formulas often without notice, so it is wise to do a strand test for each new touch up.
The No. 2 rule also ignored by all is: Read the instruction box! In particular, the time chart that corresponds to the type of hair that you have: fine, medium, coarse. Note that most people think that really kinky hair is thick and coarse. This is a big misconception. The type 4 hair is like being blonde. For your hair to curl up so much, it is in general quite fine. Do not use the "coarse" hair timing for fine hair!
The No. 3 rule is: Use real plastic gloves and not the cheap plastic gloves included in most box kits.
No. 4 rule: Are you perming your entire head (virgin hair) or just doing a touch up on the roots? Many “professional” stylists make the mistake of re-relaxing the entire head each time. This will kill your hair. Once you have relaxed your hair once, you only need to do the roots! The rest of your hair should be protected with vaseline in it and absolutely no additional coating of relaxer. It's already gone through the process.
No. 5: Read specialized relaxed hair blogs and internet websites.
There are many more rules that you can find on the internet, but these five are the musts!