Not knowing anything about green alder I'd wait until the plant goes dormant(after leaf fall) to harvest them. Then cut shoots that are second year growth(has small branches on the shoot and not just leaves). About 1/2" to 3/8" in diameter at the base. An open end wrench is a good gauge for measuring. Bundle them in 12 to 15 per bundle and set them aside for a month or so in a dry area. After that I'd remove the bark from one shoot and see if it checks as it continues to dry. If it does check wait another month and try again. If it does not check strip the bark off each shoot and rebundle for a few days and try straightening them one at a time with heat to see how they react.
I have made good arrows from all sorts of shoots I find in my woods. Some make great arrows, some make good arrows and some make good quiver stiffeners
or foreshafts for other arrows.
You could also harvest a few now, strip the bark off a some and leave it on a some and give a couple of weeks and see how they reacted.
