This build isn't exactly common. It's a rare sight to see ap miss fortunes and the difference on the gameplay of both is so huge that most players just don't get why a mf should be ap.
First off I'll explain the differences, it envolves a huge gameplay difference from most champions, MF becomes somewhat of a pusher. When playing AP MF keep in mind that your priority isn't killing enemy champions but tower-smashing and minion warding. It is your main income and it is what makes you carry. Although it isn't much straight-forward, minion warding is what makes the early game move and push an early advantage, as well as constant damage to enemy towers and almost no damage to yours.
Keep in mind that once Make it Rain is on level 5, you can kill all the minions on the area making it instant-push. And since you got nashor's tooth -25% cooldown you can spam that, so great warding. Don't expect to cause much damage without make it rain or the ult, but when you damage with it, you can burst for high area damage with both.
more will be written later.