Creating the character

Have the player pick one of the unused playbooks. They should also pick a background that reflects either a new arrival in Stonetop or a local NPC stepping up to PC status—each playbook has at least two such backgrounds. You don't want this new, highly-skilled PC to have been here all along without ever getting involved.

When the player gets to the "worldbuilding" part of their playbook (back page, left column), help them make choices that fit with the established lore of your game.

As a rule, new PCs start play at level 1, no matter how seasoned everyone else is. Don't worry about "balance." A level 1 PC in Stonetop is plenty competent, and starting fresh lets the player learn the playbook and develop their character through play.

That said: it's okay to start a new PC at a higher level if there's a compelling reason (like the PC is an ancient hero freed from stasis), the player is familiar with Stonetop, and the whole group is cool with it.