I drilled holes near the ends with my Dremel, then attached them to the table with nails. Craft wood is really soft, so it was quite easy to drill the holes. The three shelves are plenty of space for my thread collection, but I may get another one since there is room for it. I used a 60% off coupon for all of them, so they were only about $4 each.
I love that I can see what colors I have now!
The Marvel pockets are a project I made a few years ago. It had hung the entire length of the side of the desk (there is one on the other side, too). I cut the lower pockets off to make room for these shelves, which is okay since I have only been using the top pockets anyhow.