Nuts from soft-shelled, green acorns will be moister and more bitter than if mature, but are still edible. You can shake branches or climb the tree if you want, but mature nuts are eventually dropped whole or shed from the caps that remain on the tree for a while, so collecting them from under the tree daily should do. You'll have to get up early to beat the squirrels, who gather and bury great numbers. Nuts must be mashed to remove tannin most quickly, so there's no need to try and harvest the kernels whole. Smash acorns between two rocks, pick out shell pieces and keep mashed kernels.
Tannin is grudgingly water soluble.
To leach it out, it is easiest to soak the mashed kernels in flowing water. Native Americans put them in a fine-mesh bag and submerged them in a flowing stream for a week or two. Boiling the mashed kernels in several changes of water will leach out tannin faster than passive soaking. Boil till water remains unstained. A hammock spread with a sheet, kept wide open with sticks and hung in the sun works well.