I could probably write an opus on my attempts to root avocado cuttings. There are also some good old journal articles on this question, where they found that Mexican avocados rooted much more easily than Guatemalan (intermediate success) or West Indian (almost zero success), but it can take many months to root. Based on those studies and my own testing of methods, this is what has worked best:
1) Cut the branch tips right near the end of a flush (done expanding but not done hardening), removing at least 5-6 inches. This works best if the bottom of what you remove has finished hardening (firm stem) even if the new leaves at the tip have not.
2) Trim off all leaves except the smallest 2-3, and remove the dormant buds next to the lower leaves, too (roots only form on callus, not from bud sites, which might rot).
3) (optional) I now store those prepared cuttings in the back of an aquarium filter (filter media removed) until they form nice bumpy callus (a couple weeks). I lose a lot fewer doing this vs immediately going in soil.
4) Place cuttings in soil or rooting medium, buried up to just below the intact leaves, covered with a humidity dome and with bright light at least part of the day.
5) Keep it from drying out (but not soggy) until vigorous growth (6+ months). If it wilts or browns, it's dead and should be removed.
I've tried both with and without rooting hormone and haven't noticed much of a difference, both ways I'm around 50% loss rate.