Arcane Comet is normally pretty difficult to land and often unreliable, but proccing it from MF's Make it Rain slow it normally hits every time. Arcane Comet also scales well with bonus AD.

Because MF is a casting ADC, the mana sustain from this rune allows you to use many more Make it Rain and Double Up in lane to harass the enemy more.

Most of MF's damage comes from casting abilities, so getting more CDR from Transcendence is crucial to be able to use more abilities in lane and harass the enemy.

Gathering Storm is used for 2 different things. 1: Because that MF normally falls off super late game and this rune helps her scale better into late game and 2: Gathering Storm AD also scales well with MF's high lethality build playstyle.

This rune is a no-brainer. Simply brings more AD and lethality into your build.

This rune is easily abused with MF because of the slow from Make it Rain

Love Tap
Every time you attack a new target, you deal extra damage as bonus physical damage. This is cut in half against minions. Can also be procced from Double UpTaking advantage of this passive with Double Up is your main poke in lane. Bully your opponent out of lane with consistent safe damage. This passive can consistently be procced on turrets by hitting tower > creep > tower > creep > tower (repeat)
Double Up
You shoot a targeted unit dealing 20/35/50/65/80 (+85% AD) physical damage, and applying Love Tap
Normally in lane you want to hit Double Up on a creep to make the second part of this ability to hit your target as it gains extra 100% AD. If you kill the primary target the damage is increased to 150% AD!
Strut
Passively you gain 25 extra movement speed when no one deals damage to you for 5 seconds. After additional 5 seconds of not taking damage, you gain 60/70/80/90/100 movement speed instead. Activate to gain 40/55/70/85/100% bonus attack speed and gains the max amount of move speed from strut.
This ability can be used in a couple of different situations. With the crazy attack speed steriod from this ability you can make the most of your passive with switching targets to constantly proc Love Tap. It can be used to close a space between you an an enemy or to run away.
Make it Rain
Rain down bullets in a target location dealing 80/115/150/185/220 +80+ bonus AP over 2 seconds and slowing targets hit by 28/36/44/52/60% for 0.25 seconds. The slow is refreshed if an enemy stays within the target location.
This is mostly used for the slow or to check brushes instead of facechecking. NEVER use it to poke enemies as the mana cost is pretty high and doesn't really do much damage as it scales from AP. However it can be used for the slow to set up Bullet Time so it's harder for targets to escape and forcing a flash.
Bullet Time
Channel a barraage of bullets in a cone in front of her every .25 seconds for 3 seconds, dealing 75% AD per wave. 12/14/16 waves are fired in total dealing physical damage to all enemies caught in the area. Each wave of this can crit, dealing 120% damage.
This ability is absolutely devastating with the new build, as it procs every 0.25 seconds it will ramp up your Black Cleaver pretty much instantly to shred their armor. Make sure that you cast this in a safe location because it can be stopped by cc