One way to do it is to hang them from "chains". Sorry I don't recall the name, but it is a cheap, light chain where each link is just bent, not welded.
I unbent the last link in the chain to form a hook, made a loop of the chain where it passed through the fixture's hanger, and used that hook to attach the end of the chain to the hanging length. WHERE I hooked the hook controlled the length of the chain, by changing the length of the loop.
I'm sorry, that probably makes no sense at all, and I don't have a photo!
You can also just dangle the chain down from the upper shelf, and add a hook to the fixture. Just hook the hook into the dangling chain when the fixture is the height you want. But then I felt I had to tie the extra length of chain out of my way so it didn't hang down into the plants.
P.S. You have to support the weight of half of the fixture with one hand while fiddling with the hook using the other hand. Then lift the other end of the fixture, and adjust that hook.
