I've been playing with spinning, dyeing and fiber crafting for a bit now and just wanted to reassure you a bit if I could. I mostly work with protein fibers but flax and hemp make their way into my fiber stash, too. In my (limited) experience making plant fiber mordant like this is as simple as putting a couple handfuls of foraged acorns in a quart jar and topping it off with water. Give it a few days to a month and you will have tannin tea. I suppose crushing them would speed up the process but I wouldn't worry about it too much. Especially if you don't want to process them further into flour. Some of my favorite dye plants to forage for around here are rabbitbrush, cherry bark, walnuts, aspen leaves and sages.