Well, this build started when I first tried Fiddlesticks. After I saw his Drain, I immediately thought at buffing that up, constructing the champion to revolve around that particular skill. This build is meant for playing with an allied champion by your side. If you want to play mid then choose another one. I also found it useful at jungling as creeps can't really damage you because of your constant lifesteal and Drain skill.

The items are basic lifesteal/AP items. I started with the Vampiric Scepter and then went for the Emblem of Valour. This is a great item just because it gives you a decent lifesteal amount and also has a passive that gives nearby champs a nice health regen. Overall it keeps both you and your partner in the lane longer.
The next item I went for was the Bilgewater Cutlass. Its lifesteal adds up to the precedent and the basic damage buff helps at farming.
After you score some very nice early kills and/or assists early game you begin to be targeted more by the enemy champions. That's why I went for Sorcerer's Shoes for the movement speed and magic penetration.
The next logical thing to choose was the Hextech Gunblade. Why? Because it's awesome. It gives you 20 lifesteal, 60 attack damage, and also has an amazing active thats so useful in so many situations.
The Hextech Gunblade is so awesome that I decided to get two of them.
At this point in the game I tried to focus on AP so I got myself a Rabadon's Deathcap for obvious reasons and after that Mejai's Soulstealer. This last item is great if you are already scoring kills and assists and want that cherry on top of your cake.

The skill sequence is nothing too special TBH. I wanted to max out my Drain as fast as possible so every other skill revolves around this one. It's basically your choice when it comes to what gets to who and where.

The Summoner's Spells are meant to keep someone near you. Flash is for when I want to gank or I want to escape and Exhaust is for keeping my beloved enemies near me and don't let them run from my love. Ignite works good as well alongside with Flash but exhaust is my final choice.

Well, the combo I use with this build is meant to:

1)Silence and render enemies useless;
2)Don't let them escape from me;
3)Replenish as much health as you can while dealing huge damage.

Team work is a big deal when playing Fiddle (unless you come across weak champs with low HP).

My first skill is Dark Wind. Dark wind does a small amount of damage while silencing several enemies (both creeps and champions). Immediately after that I kick in the Drain and use Terrify or Exhaust to keep my prey as close as possible to me.

Fiddle's ultimate is a very very very very useful skill:

-First of all it kills creeps like crazy. If you want to push like crazy and there's tones of little bastards in front of you, Fiddle's ultimate will cut through them like a knife through butter.

-Secondly it will always be a great initiation as it creates panic through the enemy team. A well planed gank against 3 or more enemies initiated with this skill can give you both assists, gold and satisfaction.

-Thirdly it can help you escape enemies or just hold on enough until someone helps you. It has happened to me many times to be alone against an enemy that keeps pounding my ***. I used the ultimate and then terrified and drained the HP of the enemy until someone close-by came and finished him off.

Remember: The most important thing when playing Fiddle is common sense and to keep in mind that you have huge amounts of lifesteal so damage=HP for you.