Equip the Faded Ribbon and the Toy Knife, Drop the Bandage and the Stick, buy as many Spider Ciders as you can from the Spider Bake Sale, and get the Butterscotch Pie after killing the 20 enemies but before killing Toriel in The Ruins
If you want to make your journey a little easier, you'll want to stock up on some healing items and equipment:
Continue as usual as if you were playing through a Neutral Pacifist route Kill 40 enemies in Hotland and/or in the CORE before killing Mettaton Kill 16 enemies in Waterfall before killing Undyne Kill 16 enemies in Snowdin before killing Papyrus Kill 20 enemies in The Ruins before killing Toriel After you've completed your first True Pacifist route, restart your file and begin a Genocide route.
