If the fabric is too fine, it won't work to well and if it is too large, the quality of your keif will suffer due to the amount of leaf material that falls through.

Be sure and freeze your material for at least 45 minutes, so the trichomes become hard and brittle and breaks of the leaf easier. First, grate your material through a flour sifter, which then falls onto the screen. It will take a little practice until you figure out how gentle or rough you need to be. If you bounce and shake the screen, you will get purer kief, as opposed to running a card across the material, forcing it through the screen. You'll have different grades as you go.

You can then press the trichomes into hash.