The Will skill represents your character’s general level of mental fortitude, the same way that Physique represents your physical fortitude. Note that Will doesn’t add any mental stress boxes; take a stunt for that.
Overcome: You can use Will to pit yourself against obstacles that require mental effort. Puzzles and riddles can fall under this category, as well as any mentally absorbing task, like deciphering a code. Use Will when it’s only a matter of time before you overcome the mental challenge, and Lore if it takes something more than brute mental force to get past it. Many of the obstacles that you go up against with Will might be made part of challenges, to reflect the effort involved.
Contests of Will might reflect particularly challenging games, like chess, or competing in a hard set of exams. In settings where magic or psychic abilities are common, contests of Will are popular occurrences.
Advantage: You can use Will to place aspects on yourself, representing a state of deep concentration or focus.
Defend: Will is the main skill you use to defend against mental attacks from Provoke, representing your control over your reactions.