In Texas Elderberry grows as high as 20 feet. Don't know what it does in other areas.
As for preparing it. Take it to the house and remove the bark. It will sometimes get wbugs, in it.
I don't spend a long time curing white wood of any kind, it's a waste of time.
I would split the log in to staves. You may get as many as 4 from that log.
Then I would work them down to stave size, clamp or tie 30 of them to a 2x4 to keep them straight. them bo ahead and make a bow out of the other one.
Keep the dimensions the sames as those gorm any other white wood bow. If you need ideas on the PM me.