It's easy enough. In a hive you just do a tap tap tap on the side with a pocketknife or a hive tool. Not hard. Just rythmic and steady. The bees move up. In a tree it may take more to resonate but you can listen and tell if the tree is resonating. You may have to find a thin spot (where the hollow gets closer to the bark) to get any resonance if you want to move them up in a tree.
You can speed the process with some smoke.