You buy a Bentley manual for the mechanic's how to of working on the car. It has torque specs and walk throughs for every project. Then you have BMW forums and helpful people who have done DIYs for you. I've learned by having a friend who was knowledgeable, a car that I'm willing to tinker on and lots of internet research.
Youtube can be very helpful, pelican parts has a good set of DIY write ups that mostly focus on e36, but I think a fair amount translates.