Identify the square and circle marks on the top and bottom edges of your panels. Line up the squares on a rear panel and a left side panel. Slide the left side panel into the grooves on the rear panel until it clicks and the two panels form an L shape.
Line up the circles on the edges of the rear panel and the right side panel so that the three panels form a U shape. Slide the right side panel into the grooves on the rear panel until it clicks.
Slide the shelf into the desired slots between the two side panels.
Increase the height of your rack by building a U shape with one rear panel, one right side panel and one left panel as explained in the previous steps, and set it on top of the first U shape. Align the posts on the bottom on the second U shape with the slots on the top of the first U shape. This will lock them in place.
Expand the width of your rack to the left or right by aligning the the squares or circles on the first and second rear panels and then sliding the second rear panel into the grooves on the first panel. Attach the side panel by lining up the square or circle marks and sliding it into the grooves.