The weight of it alone keeps it aligned but to make sure I used the cigar/tube shaped shelf pins. I put one on each side and I think 2 or 3 on the back. I marked all the spots on each section to put a pin. I then took the sections apart, measured a set distance ( I think 1/4") in from the outside edge and drilled the holes. I put the pins in on the lower section, put the next section in place, and repeated.
I wanted to have the shelf pins in the exact same spot on each section so that any section could be the bottom or middle. However, I ran out of time so I just had to install them as I could.