MF is an extremely cheese pick, but I have had great luck with her. A lot of the viability is based off your opponents underestimating you. Let them make the mistakes and make it rain victory.
I personally find MF a ton of fun and it can be fun not playing champs in their typical role.

In lane it is all about zoning with your e.
I know most people start with q and go down the line but e is your most important ability especially early so lets talk about that one first.
When you get to lane, the enemy bot lane is going to doubt ap mf support.
Poke them with your e when possible by placing it where they need to go to farm. Watch your melee and caster minions and wait until they get low and you know the enemy adc will try to last hit them and just throw your e on them. Not only does the damage stack up over time, it also slows. You can auto them if they try to disengage or back off, or just simply walk away and do it again next time your e is up. When your e is down, be very careful and do not get engaged on by heavy engage supports (Thresh, leo, naut, etc)

Your q may be used to poke (especially for gold) by targeting a caster minion and allowing the shot to bounce onto the enemy. No where is safe against ap mf.

Your w is fairly irrelevant. While it does do damage in extended trades, it doesn't have much of a use for you.

R will win you teamfights. The most important thing about this ability is patience. When a fight erupts after you have ult, note which champions on the enemy team can disrupt your ult (knock ups, stuns, fears, etc.)
Ideally, wait for those abilities to be used and position yourself in a safe spot to ult as many of them as possible and let it rip. Your ult applies your w stacks so there's your purpose for that.

More coming guys, please be patient.
I'd love to hear some feedback.