I've carved a week into canon at the beginning of STIII, between the return to Earth and the meeting in Kirk's apartment. And I've tried to stick to canon as far as possible with some original flashback material from the oh-so-useful second mission post V'ger.
With huge thanks to Djinn for her feedback, input and support.
This is what he knows.
He knows how to gentle a horse with his hands, and how to steer a shuttlecraft though an ion storm with his fingertips.
He knows every intermix formula in Mister Scott's repertoire (plus a few more purely theoretical options he's boned up on, just in case). He knows the sweet spot on deck six where he can hear through the soles of his boots when they've got it just right.
He knows how to spot the ensign who's about to have a crisis of confidence, and push them just far enough to spark, but not so far the light dies.
He has learned how to detect when a Tellarite is uncomfortable (the nostrils being the window to the soul in this case) and how to wrap a gift for a Deltan so the recipient can delight in the unwrapping (fur is preferred, silk acceptable).
He knows three ways to cook Katarian eggs, he knows he doesn't have the patience to meditate and that he'll never understand the concept of a hobby.
He knows that when you're attacked by a larger opponent the secret is to make yourself small, to use their sense of superiority against them, to trust your body, not your brain. He knows how to take the pain from a well placed blow and bury it away so it becomes a thing apart.
All this he knows.
But he doesn't know how he's going to get through this. He doesn't know how to wake another morning to the thud, the punch in the gut, the knowledge that Spock's gone. He doesn't know if he can ever step onto the bridge of a starship again and see that empty spot and command a crew with the better half of him missing.
He has a lifetime ahead without Spock and he doesn't know how he's going to get through the next week. Hell, he doesn't know how he's going to get through the next five minutes.