I discovered Ms fortune while i was playing Sona, eachtime those games where devastating. It turned out Sona and Ms fortune make the best combination together. But looking from the Sona perspective and analyzing those games, i came to adjust how Ms Fortune can be played and build.

Mainly Ms fortune is a marksman played not on frontline, your a bit weak, but your E is powerfull and keeps enemy away. Based on that i created this build, Ms Fortune is like someone who fires canons from the back (E), and peeled enough you run front and kill Q or R

Well i'm writing this because this is a nice way to play here.
Most think of marksman as a champ to target and shoot and kill or so.
Ms Fortune's E, make it rain; is really here best part in laning.
And since she is kinda a lane king, a good champ to lane with.
I rater focus on that, instead of hoping you targeted correctly with your Q.
Go for easy kills and keep pushing with E.
And since you build is based to getting yourself mana, you wont run out of it.
You can do this, while your opponent marksman probably goes for straight damage items.
But he can use them less because even those will cost him mana to fire.
Also the pushing effect of E will pretty soon push them to their turret
Where their turret kills your enemies (so you get a farming advantage)

Its excellent if your with a support champ that also can do some positional locking or slows, because once the bullet rain has fallen (and you can cast them in a long range), you have enough time to run to it, and still do that kill Q shot directly or indirectly via a minion (or champ)

Besides ms fortune is a nice champ if you can stay away from the enemy and well E is better then Q to keep distance. And despite the 0.5 sec before it is activates that still is fast enough to surprise your enemy, who you slowly peel with it.

Once you get R then more easily do E + R for the easy shot.
R is also great with a support that can lock others in place ofcourse.

And a final note she works best with an aggressive played Sona (she is not there to heal, but to poke) you both can peel them to death in this setup

E is also nice to do when the enemy team lane champ is surrounded by minions, and cant get easily out.

And.. your masteries will do just that extra damage while pressing E for any champ who stands to long on it.

When your teaming over the terrain, its again your E that is very usefully.
While you might not survive a close combat your team members probably can, while your E helps them to get that champ, and occasionally might give you that kill too.

In cases they are in a team fight, you from a distance might use R as well to finish faster

As you can out run some other champs, so that might become handy in those secret-op operations where you run for kill, or get them behind you and so your team can push that other lane, speed is nice.

Pros

Passive gain speed
Your Q Shoot, and shoot trough and hit behind
your W can give you a little speed for example to fire faster on turrets
Your E is excellent to push lane, slow and peel from a distance, while you stay safe
Your R kills


Cons

Once the enemy gets you alone close you got a high risk of dying, on short distance your weak
You should stand still when hitting with R, or else it ends