Look for rectangular holes at discrete intervals along each rail. The divider has at each end a spring loaded rectangular shaped rod that will fit into the holes. Once engaged, you have to squeeze the two stubs that stick up on the top left side of the divider to disengage. The spring loaded rod ends do not easily engage the holes, when you get the divider near the hole set you wish to use, you kind to have to jiggle it a little. Once engaged, though, they stay put. I like mine a lot, especially like the feature of being able to swing it up horizontal to the bed so you can slide in long objects. And by the way, if you figure out what to do with those eight silver pieces of metal, please let me know. That stumps me!