I've found the best way to collect seeds from a plant is to shake the plate the buds are laying on to be burped. If you pollinated heavily and the bud are ripe seeds will fall from the buds onto the plate. Once you remove the buds from the plate you'll be able to seeds on the plate
Also if you have a bowl trimmer seeds will pass through the screen into the bottom with all the other larfy material. Kinda the same approach starting in order to separate seed from the bag of larf is put some of the material on a rolling tray then card the material several times.