1) Talk to Hetty, Tell her you are looking for work. She will ask if you want to become more proficient in dark arts. Tell her you are in. She will now say that we are to make a potion which requires: Rats tail, an onion, burnt meat, and eye of newt.
2) To obtain the onion run North to the onion patch and pick an onion (you can also pick garlic here for a later quest). The eye of newt can be obtained from the magic shop in Port Sarim which is located East of Rimmington. Burnt food can be obtained from burning any type of food. The rat tail is obtained from level 1 rats located in the Varrock Sewers, go near the north-east fountain, run north and there is a sewer grate, go into that grate and that's where the rats are are. kill a rat and loot it for the rat tail.
3) After Gathering all the materials make your back to Hetty in Rimmington. Speak to Hetty, She will ask if you have gathered the materials. Say yes and hand her the materials and she will tell you to drink from the cauldron now. Take a drink and she will say that you should now be more experienced in the magical arts and reward you with 1 quest point and 325 runecrafting xp.