Dice the plant material (in this case, onion) into small pieces. Use a chopping board.
Put your diced plant material into a mortar, add a spatula of sand and crush with the pestle.
Put 5cm cubed of alcohol to your crushed plant pieces. Make sure there are no flames around alcohol because is flammable.
Carry on crushing the material, taking care not to spill any alcohol. It may get slightly tiring!
place a funnel in an empty test tube and fold a piece of filter paper so that it fits on the funnel. then pour it in your well crushed and mixed mixture in to let it soak through into the test tube.