What are your goals?
Think about what you want this move to add to the game. For example, your new move might…
... grant permission, like how Spirit Tongue lets the Blessed talk to spirits and natural beasts.
... establish how stuff works, like how the "attempt to commit violence on the roads" move Custom moves says what happens when someone does just that.
... encourage certain behavior, like how Better Part of Valor pushes the Would-be Hero to avoid fights.
... establish risks and stakes and serious consequences, like how the "fall from the boughs of the tree" move says just how bad it'll be.
... streamline play, like how Outfit) and Have What You Need) make packing for a trip easy.
... embellish parts of play, like how Keep Company fills in scenes that might otherwise be glossed over.
... shape or frame play in a particular way, like how Burgle deals with a Fox sneaking off on their own.
... reinforce a theme or mood, like how Haggle makes deals with the Fae always feel fraught or how Drowning creates tension as PCs hold their breath.
... get players to buy into adversity, like how the "call of the nosgalau" move has players say what compels them to leave the roads.
... push players into making interesting, meaningful decisions, like how Forage has players pick between food, a discovery, and safety.
... explore a PC's feelings, thoughts, or past, like how the "Hall of Humility" move has them reveal a moment of humility or one of pride.
... prompt you to think more about the world/characters, like how Persuade vs. NPCs forces you to consider whether an NPC has reason to resist, and what could convince them.
You'll probably have an instinctive sense of your goals when you start writing (otherwise, you probably wouldn't have thought, "I should write a move for this"). But as with constraints, it can help to name them and consciously design towards them.