Wow, this is funny. I just did this today. I normally use Phyto 1 with long stretches in between (5-6 months). Today I used Phyto 2 do a corrective after a 3 month stretch. This is going to be long, but instead of do's and don'ts, I'll just tell you what I did and why.
Basically, Phyto 1 barely changes my texture. It just softens and loosens my curl pattern, but there were still tight 4a curls in my crown area. This was causing a breakage issues because I had three textures - my new growth, my texlaxed hair, and my straighter ends. I decided to go up a strength to even it all out. Not to say that a relaxer can fix breakage (it can't), but my hair was breaking a bit in the middle right at the line of demarcation where the straighter hair met the texlaxed hair, kind of like a person who has breakage during transitioning.
So, I did the half & half method. I used Phyto 2 and didn't mix anything in the relaxer. I based my scalp, etc. with Vaseline & used the pre-care balm that comes with the kit for my ends. I also carefully detangled my new growth first to make sure I could part my hair quickly. (Detangling dry new growth was the hardest part - normally I would have straightened my hair the week before, but I was working out a lot). I applied the relaxer on the bottom half of my hair. I spent 8 and a half minutes applying and about 5 minutes smoothing. As I smoothed row by row, I added more relaxer onto the parts that were underprocessed. For the last 2 minutes or so, I continually smoothed my hair all together, trying to make sure that I got all the under-processed parts. So, basically, I did my unprocessed newgrowth first, and then intentionally overlapped to correct the underprocessed parts. (The relaxer was on my hair a total of 15-16 minutes).
I rinsed, neutralized, and repeated this process on the top half of my hair. I then carefully neutralized my whole head over again until there was no pink, then conditioned, etc. If I could do something differently, I'd do the top half first because rinsing was messy.
This is my first time doing the half and half method. From now on, I think I will only use Phyto 2. There was no burning and my hair still had some texture to it, which is great. Doing a corrective is hard and it's best to have a professional do it. I don't trust anyone else in my head anymore, though. When you do a corrective, relax the newgrowth first and then the part you want to correct.
I also trimmed my hair about an inch & a week before I did a hard protein treatment with Aphogee. (Actually, it was probably less than an inch)