This is covered under the move Craft a Spell. Essentially, you determine how many points you get by how often you anticipate making use of the spell, and then add an extra point for each potential mechanical consequence for rolling a weak hit or miss (such as a loss of health or momentum.) You add these together to determine how many points of spell creation you get for that particular spell, and then spend them on the benefits that you gain on a hit when you use that spell.
I hope that helps.