Once you're all set, log into RuneScape and select Fresh Start Worlds Event.
Old School RuneScape
To join Fresh Start Worlds in Old School, you’ll need to create a brand new account and complete the tutorial on any world.
At the end of the tutorial, before going to Lumbridge, select that you'd like a Fresh Start account. If you're not already playing on a Fresh Start World, the game will transfer you to one, where you can begin your journey.