Witch's Potion

Hetty the Witch
Talk to Hetty in her house in southeast Rimmington, and tell her you want to know more about the black arts. She will tell you she is making a potion and needs four ingredients that she wants you to get: a rat's tail, a piece of burnt meat, an eye of newt, and an onion.

Rat's tail
You need to start the quest to get a rat's tail. Kill a level 1 rat (they can be found in the archery shop west of Hetty's house) to obtain a rat's tail (it may take a few kills).

Burnt meat
Kill a giant rat just north of the chapel graveyard southeast of Rimmington, or kill a cow in the cow pen just south of Falador, and cook the meat. There are ranges where you can use to cook the meat, just south of the magic shop in Port Sarim and in the house north of Hetty's; you can also light a fire yourself to cook the meat on (logs spawn in the general store of Rimmington). If you didn't manage to burn the meat, use the cooked meat on the fire or range to purposely burn it. You can also buy raw meat from Wydin at the food store in Port Sarim.

Eye of newt
You can buy an eye of newt from the magic shop in Port Sarim for 3 coins. The magic shop is labelled with a fire rune symbol on the map.

Onion
Onions can be found in a field northwest of Rimmington (directly east of Melzar's Maze) or in the backyard of Fred the Farmer's farmhouse west of the sheep pen in Lumbridge.

Finishing up
When you have all these items, talk to Hetty. She'll put them in her cauldron and tell you to drink from it. Drink from the cauldron to complete the quest and gain your reward.

Rewards

 * Quest point
 * Magic experience