Step 1
Go to Rimmington.
Talk to Hetty.
Get onion - field near Melzar's Maze.
Kill rat in Rimmington, collect tail.
Collect meat (Cow, Bear, Giant Rat).
Cook it.
Cook again to burn.
Return to Hetty.

Go to Rimmington, located south-west of Falador and west of Port Sarim. Go to the south-eastern house, which belongs to the witch, Hetty. Talk to her, and when she asks you if you would like to learn the dark arts, answer yes. She will tell you that she needs an eye of newt, an onion, a rat's tail and a piece of burnt meat.

To get the Onion, go north-west of the Witch's house to Melzar's Maze. You will find a field at the front with onions in it. Pick one.

To get the rat's tail, go into the Arrow Store, next to the Witch's house. Kill a rat and pick up it's tail. You will not be able to get another rat's tail after completing the quest.

To get the meat, you must kill a Cow, Giant Rat or Bear and cook the meat on a range. If you successfully cook the meat, you can use it with the range or fire again and it will burn.

For the eye of newt, you can go to Betty's Magic Emporium, which is in Port Sarim, east of Rimmington. Right click on Betty and click Trade. Buy an Eye of Newt, with your 3GP.

Return to Hetty in Rimmington, and claim your reward.