Okay, to start of with this is my first guide. You guys might not like it and I don't blame you for that, but this is my way of playing Fiddlesticks.
I may have stuffed the skill sequence up a tad but who uses that anyway?

This guide will help you dominate a 3v3 with Fiddle, and hopefully stay on 0 deaths. Most of the time it works for me, and I wish you luck.

This guide will be brief but effective.

Okay the runes are pretty self explanitory and will help you early game. Though the mana regen may come in helpful lategame also.

Greater Mark of Insight - Helpful for early game Magic Pen, dominate the enemy with Fiddlesticks' abilities.

Greater Glyph of Celerity - Good at Level 18 with the CDR. (Cooldown Reduction) Really helps a lot.

Greater Quint of Insight - This one is optional, though I use it for even more Magic Pen early game. Really good for getting fiddle fed.

Greater Seal of Replenishment - I find that using the build I do, Fiddle is constantly low on mana and has no regen. So this set of runes will help greatly during the game.

Okay so firstly, fiddle is best when kept in surprise until ready to strike. Most of the time I'll wait in brush with fiddle until an enemy champion is in range. (This is 1v1)
I'll pop my ultimate, (Crowstorm) Ignite them, Fear them, and then drain. The fear will hopefully keep them inside your Crowstorm's range, while you drain them as they're ignited, doing maximum damage, if this fails, they should at LEAST be on extremely low HP, so use Dark Wind to finish them off. Unless you're trying to take on a magic resist take then chances are this combo will finish anyone off, or severely damage them.

So start of with Doran's Ring for the Extra Regen, Damage and Health. Stay in the lane until you have say, 750 gold, by which time you should B and get Boots of Speed. Return to the lane and farm until you hit either 400g or 1325g respectively, and buy Catalyst The Protector, or you could stay laned until you have enough for this and Sorcerer's Shoes.
Once you have those either recall at 860g or wait until you have enough for the Rod of Ages. (This item is essential for fiddlesticks in my opinion, it stacks over time so be sure to grab it early.)
Next you'll want either Giant's Belt for the extra HP or Blasting Wand for the AP. I usually go Giant's Belt. Farm enough until you have enough for Rylai's. If you're doing really decent you'll definately want Mejai's Soulstealer.
Your next item should be Rabadon's Deathcap, because with Rylai's and Rod of Ages your AP will be about 150 - 250 (Not sure on the exact amount) and you'll need that extra AP for the damage. If you feel it's necessary to avoid Morello's Tome because your CDR is fine, and you've stacked Mejai's, I suggest getting a Void Staff or the Abyssal Scepter, for the Extra damage.