I pick the flowers that have just popped and the stamen are now releasing pollen. I get a bunch, 20 or so, in a little plastic measuring cup and bash them a bit gently and all the pollen falls out. Then it's easy pick out the flowers and you're left with the pollen. I got wax paper, and made little piles of pollen on the wax paper, then folded it over and sealed them with masking tape. Into a ziploc sandwich bag, and into the freezer.