What is the guidelines or set of rules that I can use to build a custom paragon path for a character, and still be balanced?
Get to know the existing paragon paths well. There are lot's of them though, so it'd be a fair bit of research to read all 400 or so of them!
Play a bit at paragon path levels and sample some of existing paths out there.
Write you path and select similar bonuses and level of effect as other paths.
Play test your new path.
Tweak based on feedback.
Play test it again.
Get someone who hasn't DMed it or played it to then play test it again.
Tweak a final time, it should now be fairly balanced.