Fill a used compost bag with comfrey and nettle leaves. Tie it up and pierce it repeatedly with a garden fork. Put it in a water butt and fill with water. Leave for a month. Take out the bag of leaves and discard. Dilute to roughly one part feed to 10 parts water.
Later in the season when nettle is no longer available then comfrey and borage is a good alternative.
Use a measuring jug to fill used 500ml bottles which can then be added to a watering can as and when needed.