You can buy an eye of newt from herblore shops. Greenman's ale is sold in Yanille. You can buy a rotten tomato at the Duel Arena or from the crates near the pillory cages in Varrock, Seers' Village, or Yanille. You can make a fruit blast at the Gnome Stronghold in the cocktail bar (Blurberry will give you a cocktail book on how to make them). Use the ashes on the fruit blast to get a dirty blast. Once you have all 4 items, give them to the Cook. Congratulations! Quest complete! (for now...)