I don't know about manzanita specifically, but most branches are safe to use right away, but it is a good idea to wash them first. And for branches with cracks or rough bark, ect., it's usually good to also bake them in the oven at a low temperature for a short time to kill any insects or insect eggs in the wood.
As for seasoning - it might help to let the wood dry out before working with it to avoid splitting, although I've never had a problem with it, when working with softer woods like maple or apple. Manzanita is such a hard wood to work with, it might be worth letting it dry out for a few weeks before drilling it and such, so that you don't put a lot of work into it only to have the wood crack when it dries.