I've learned a lot from this and other tuning-related websites and decided to put some of this information into a guide.
This guide is centered on VP37/EDC15VM+ cars, but many concepts like linearisation, diagnostic extension, IQ limits and such are applicable 1:1 on PD cars.
It also contains a section that deals with hardware changes - what's necessary to reach what power level, stock hardware limits (for VAG 1.9 TDI cars), original files, damos, and many other things.
However, it is not a guide to copy/paste from - you'll have to think for yourself

After all, it's supposed to make you learn something!
The guide isn't 100% finished, nor will it ever be - there's always more info to be gained or included. I'll keep this thread updated when I release a new version.
Comments, corrections and additions are very welcome!
BTW, you can also find the ori file the guide is based on along with a cheat sheet (all map addresses) in this thread here on ecuedit.
Kind regards