Should you define a 6- result?

Most moves don't say what happens on a 6-, which means that the player marks XP and you make a hard GM move—one that flows from the established fiction and that follows your principles.

Write a 6- result for your move only when you have a good reason to do so, when you judge it to be preferable to making a hard GM move on the spot, and when you can be sure that the result will make sense whenever the move gets made.

For example, Death's Door tells us what happens when a PC is dying. On a 6-, the PC either dies or clings to the world via undeath—which frees you (the GM) from having to decide the PC's fate.

Meanwhile, Struggle as One's (page 328) 6- limits you to a specific GM move, put someone in a spot. It has you zoom in from a group action to a specific situation, which is then resolved by either another player's 10+ or by playing out the scene.

As a final example, Trade & Barter (page 540) says "don't mark XP" on a 6- and the PCs can't get or sell the item here and now. It's intended to be low-risk with a high chance of failure. The 6- result both establishes and reflects that lack of risk nothing risked, no XP gained.

What should be added to the roll? Most moves have the player add a specific stat (STR, DEX, etc.). See Stats for guidance on what each stat represents.

If different stats could apply based on how the PC approaches the move, then provide guidance on when to use each stat, like how Defy Danger does. Or just say "roll +STAT (per Defy Danger)".

If the move reflects the members of a community doing something, rather than a specific PC, use one of the community's steading stats instead Steading stats.

Another option is to build up a modifier based on things that are true, like how the "stand at the Crossroads" move has the player add +1 if they bring a possession of the deceased, another +1 if they bring food or drink that the deceased loved in life, and a final +1 if they are tied to them by blood. This approach takes extra time to resolve, but it can encourage players to engage with the specifics of the fiction. It also works well for moves that apply to everyone in the party, or those that resolve the actions of NPCs (who don't have their own stats to roll).

Your move could have the player add a held currency (per Hold and spend) in lieu of a stat. No standard moves in Stonetop do this, but it's a valid approach!

Finally, your move might have them roll +nothing, with no modifier at all. Death's Door takes this approach to make the move feel impersonal and arbitrary.