Part 1 is just for use on the rads. Don't use it on anything else. Part 2 is for when you have connected everything back up and cleans out everything, it also neutralises any remaining part 1 in the rads. Follow the instructions to the letter. There is also a video guide for each part
here.
To be honest I have had mixed results with the blitz kits. Part 1 is excellent for cleaning out new rads but I found that part 2 left any gunk it picked up in the blocks which I needed to strip down and clean by hand anyway. Now that I have found out how good (and cheap) Mayhems tubing is I won't be buying anymore Blitz kits and will just replace the tubing and strip the blocks, pumps and res down and clean by hand which I have had to do after part 2 of Blitz anyway so I will save a day of downtime. For rads I will go back to white vinegar and hot distilled water.