Witch's Potion

Starting out
Talk to Hetty in Rimmington. She asks for 1 burntmeat, eye of newt, an onion and a rat's tail, and in return will increase your knowledge of the dark arts (Magic).

Tail of Rat
Head out of Hetty's house and enter the building to the west. Kill a level 1 rat. It should drop a rat's tail, as long as you've started the quest.

The Onion
Walk north a little way and you will find a patch of earth with onions on it. Take one.

Eye of Newt
Head east to Port Sarim, enter the Magic Shop and speak to Betty. Buy an eye of newt for 3 coins from her store. If there is no remaining stock, wait for the stock to replenish.

Burntmeat
If you don't already have raw meat, kill the giant rat south of the Port Sarim jail and cook the meat it drops (there's a range in the building north of Hetty's house). If you don't burn it, and get cookedmeat instead, use the cookedmeat on a fire or range, and you'll get burntmeat. You can also get burntmeat by killing Imps.

Finishing Up
Head back to Hetty and give her the four items by talking to her. Right click the cauldron and select drink-from.

Congratulations, quest complete!

Rewards

 * Between 275 and 5,175 Magic experience, depending on your level. (225 + 50 * level)
 * 1 Quest Point.