The only way I picked up on all this knowledge was to read all I could, and try what I understood. If I had problems, I had to do a bit more reading because most often that problem I ran into was already posted and discussed on a forum someplace.
Here is a good place to start
[GUIDE] How to Install a custom ROM *NOW WITH VIDEOS* - xda-developers
Also, there is a certain amount of confidence put into custom ROMS. I had a few times where the ROM would not flash and I had to use the ODIN one click to return to eclair 2.1 (JF6) and try once more.
In addition, with custom roms, there will be hidden problems which might not be apparent in the few minutes after flashing the rom. Review the rom thread and use the search, most often that problem has been reported and discussed. Read the thread to look for answers.