I have been using this build for a long while now and since DFG is going away next patch I thought why not it has been successful.
I have been using this build on ahri since I started playing her and at the time of me playing her I was never used to actives the only one i probably was fine with was zhonya.I also find DFG not needed since her E grants 20% dmg increase to all of ahri's moves on that target.

My runes are used on all my ap champs these runes just do a lot of poke dmg and early game ap helps a lot with ahri's true dmg at early levels.
You do not have to follow my runes those are what I use all the time.
I suggest use runes you are comfortable with that works on all your ap champs.

Since ahri's main attack is her Q you would want to max that out first but have your other moves to help burst down the opponent.

I suggest using E to start the fight in a 1v1 situation or to get a important target out of placement in team fights to burst them down.

Your W is useful when you are getting your last few hits on a target that was hit with your E since it auto chases champs and after the first fox fire hits if the others hit the same target it does extra dmg.

When farming in lane you would want to hit minions with your auto and poke the enemy champ with your q when you see a moment where they are either:
1.About to auto a minion
2.trying to position themselves for a poke on you
3. Their movements are being predictable and you can throw it ahead of them and make them run into it.
(I would do the third option all the time cause i find it amusing.)
you would also want to use your q to hit minions to build up stacks for your passive.
If you want to push,use your Q after you kill a few minions with a last hit

Thanks for Checking out my first moba build if you have any suggestions to help make this better message me I am cool with tip from others.