Talk to Hetty in Rimmington, and you will ask for work. She will ask you if you wish to become more proficient in the "dark arts". Accept her offer, and she will tell you that she is going to make a potion to "help bring out your darker self". To do this, she will need four ingredients: a rat's tail, some burnt meat, an eye of newt, and an onion.
Eye of newt: The closest source of an eyes of newt is Betty's shop, which can be found in Port Sarim, marked by a magic skill icon on the minimap. Once inside the shop, they can be purchased for 3 coins each. Alternatively, they can be bought on the Grand Exchange or in a Herblore shop.
Onion: The closest source of onions can be found in a field just North of Rimmington, and can also be found in the field behind Fred's farmhouse to the North-West of Lumbridge. Alternatively, it can be bought from the Grand Exchange.
Once you have gathered all of the ingredients, return to Hetty in Rimmington, and she will put them in her cauldron and tell you to drink from it. You will do so, and you will become more skilled in magic.
Prior to 24 June2010, Witch's Potion was a quest. Due to an update, it was relegated to be a miniquest. The reward of 325 Magic Experience did not change.
Prior to 7 September2010, your character would say to Juna "...And once I got her all the ingredients, Hetty's potion increased my magical power!" to which Juna would reply, "I see you are on your way to becoming strong in the magical arts." Because this is no longer a quest, it was removed from the list of adventures that you would describe to Juna. This is unlike Sheep Shearer, which the player can still mention to Juna.