on the left click Publications
Then Cick THE STRAIGHT DOPE EDUCATION SERIES
Now the 7th item Getting Off Right is a 2.9MB .pdf all about injecting drugs and how to to it safely.
This is the guide I used when I started, and I can say that it has kept me healthy and free from even a single infection.
I haven't seen this posted since I've been here so I figured some might benefit.