I have customized my own build of fiddlesticks because no other build of fiddlesticks suited me.
let me start with the mastery tree, with OFFENSE:
a offense of 5, 3 points to brute force, 1 point to mental force and 1 point to demolitionist. why? summoner's wrath is useless for teleport and flash, butcher does so little difference to help basic attack. Brute force helps attack damage, i only need 1 point for mental force so i can get demolitionist. I find after demoltionist, offense does little help compared to:
DEFENSE:
I like high defense, high mana and high health to help with fiddlestick's abilities. a High mana, to "spam" his abilities, his high health and defense to help with onslaughts of enemies.
The mastery tree of defense, as you can see i have given 21 points, why because in games i want to live long (high health, high defense)and play hard (high defense and high Health) + all 4 skills, terrifying people, then draining. Dark wind to inflict damage on multiple enemies and crow storm to damage many enemies. Understand these abilities of making enemies unable to attack or defend, this is when drain is most useful, the point is these skills are very useful.
RUNE BUILD:
With 3x greater mark of intellect, i get 17 mana.
With 9x greater glyph of intellect, i get 101 mana.
With 9x greater seal of fortitude, i get 48 health.
with 9x greater quintessence of intellect, i get 337 mana.
as you can see a total of 455 MANA and 48 HP.

Try getting items which include some form of AP or MP so Fiddle can get more strong.

-CORE-
Rabadon's Deathcap - You need the AP and it's passive just adds on to the AP you are stacking, really devistates enermies if you get it early. Try rushing one of these if you are doing well but if not it may be wise to build a Rod of Ages, Rylai's Crystal Scepter for a bit more health.
Zhonya's Hourglass - You do get targeted and having more armour increased your duribility in team fights. Activate Zhonya's when getting targeted by multiple enermies. Depending if your team fights are early than late you may want to buy this before Rabadon's Deathcap

-SITUATION-
Archangel's Staff - only if you have mana problems
Moonflair Spellblade - other team are heavy CC get this and possibly mercury treads
Will of the Ancients - I only get it if my team are heavy AP because of its aura, but still helpfull for Drain
Thornmail - if other team is heavy DPS but Zhonya's Hourglass works fine


-OTHER BUILDS-

CDR BUILD (35% CD)
Ionian Boots of Lucidity
Morello's Evil Tome
Rabadon's Deathcap
Zhonya's Hourglass

ANTI TANK BUILD
Sorcerer's Shoes
Rabadon's Deathcap
Void Staff
Deathfire Grasp
Abyssal Scepter
Zhonya's Hourglass

TANKY FIDDLE
Mercury's Treads (your choice, really only for early game)
Rod of Ages / Rylai's Crystal Scepter (HEALTH)
Rabadon's Deathcap (CORE)
Zhonya's Hourglass (CORE)
Force of Nature / Banshee's Veil (MR)
Abyssal Scepter (MR)

Pros / Cons

-PROS-

Great Disables
Crowd Control
Good 1v1 most champs

-CONS-

Easy to counter
Gets targeted in team fights
Relies on early game
Gets bullied by

Your Abilities

-MAX ORDER-

Crowstorm
Drain
Dark Wind
Terrify

Depending on what you need you can keep a balance between all spells. For example i may put points in Terrify if i find that when i drain the enemy is still escaping

But always max your ULTI since it is the most devistating in team fights

How to use Crowstorm

Never Ult in front of your foes, this gives them the time to react, where they will run or stun you
Use walls and Fog of War to conceal your self giving the element of surprise
Try to ult into the center of the other team, for this will cause them to become confused
You will get stunned if you don't cast Dark Wind in teamfights after you ult item

JungleSticks

-Jungle Path-
Blue Buff -> Wolves -> Wraiths -> Red Buff -> Golems -> Look for over extended lanes / lanes in trouble -> Recall ->Clear out Jungle till you hit 6 then roam.

Ask for a small leash at blue buff and possibly someone to guard you.

If you get counter jungled, flash away and ping so your teammates can cut off the intruder.
If your blue is gone, don't stress out, head over to his jungle and gank him and steal some of his creeps, the other jungler would have wasted time and experience coutner jungling you.

But if you do not want to risk it better off going to wolves -> wraiths -> red buff -> gank until your blue buff respawns. You may need to recall since you won't have the mana regen.

Team Work/Ganking

-GANKING-
When you get the chance to gank, after you get red or at 6, remind the peorson in your lane your ganking to let the other champion to push. When the time is right ping on who your attacking to notify your teammate your attacking and if you have Crowstorm ult in to cut off their escape or if not run in when your teammate occupies the enermy giving you the crutial time to Dark Wind Terrify and Drain. This should give you first blood or else the enemy in that lane would have to recall and waste flash, allowing your teammate to push.

When ganking mid you can stand in the brushed to the side the other champion is closer to. Or you can ulting over the walls between your tower and river.

Ganking bot or top you want to be able to get into the brush in the river and ulting when they overextend.

If the brush is warded back off and come back in 3 minutes.

-TEAM FIGHTS-
I recommend you stay out of the initiation of team fights due to all the stuns and spells being casted, wait until most of the spells have been used (a few secounds into the fight) then ult in.

It is best to not have the opposition team know where you are so the element of surprise when you Crowstorm over that wall will catch the enemy off guard. Next you want to pick a target (The guy that screws everything up for you) Dark Wind Terrify Drain them. If you are getting focused and are in a narrow area it may be wise to activate Zhonya's Hourglass just as you ult in.

Once you have casted your moves you should run away if you have low CD until they are ready, or cast your Dark Wind at the remainding targets.