Reach Combat Level 24
Unlock the Slayer menu by speaking to Maddox the Slayer in the Adventurer’s Hub
Complete enough Slayer quests to unlock Auto Slayer as a feature in the Slayer menu
Purchase or enable Auto Slayer from Maddox’s Slayer menu if available
Use a Slayer quest and keep the required materials in your inventory so it can auto-start after completion
Make sure you have the required Slayer level and quest tier unlocked for the boss you want to auto-slay