There are many ways to build a strong scarecrow and this is the build i use when i am jungling with fiddles. The deathfire grasp may come off as odd, but the reason that i chose this is for the GP10 item kages pick that helps with gold if you are having bad ganks. This character is made to combine the moves into one deadly attack made to devastate all enemies in his path.

Fiddlesticks is one of a few champs who doesn't actually need smite. You can use it to increase his clear times which for fiddle i personally don't do but i recommend for new fiddles. start with the blue buff which proves no challenge for fiddle as he can drain it to stay near full hp. Move on to wolves then to the ghosts and golems. Be sure to watch the lanes for good/needed ganks which you should be able to give them decently after you buy boots and have terrify. Don't forget that fiddles is rather slow so if they have to much distance from your gank spot then there is a very good chance that you won't be able to CC them in order to take them down. when ganking opponents with a higher % of hp left open with deathfire grasp's item use. If they are at full it will take off about 30% of their HP immediately allowing you to then CC and drain giving you a distinct advantage over your opponents. Later in the game when you have the hourglass be prepared to ult into a group and activate it to kill them but stay relatively safe.

There are many alternatives to the build i have chosen, however some of these items are considered "staples" in his build. Deathfire is useful however you can build the amplifying tome into a will of the ancients which not only helps your allies who use ap but allows you to replenish your health via your ultimate: crowstorm. You could build the cooldown boots to help your moves comeback quickly however the spell penetration allows you to do more damage to the people who have magic resistance in their runes/masteries. Deathcap, the crystal scepter, and the hourglass should stay in your build as they are the most useful items on a fiddle. the abyssal scepter gives you magic resists as well as an aura to reduce the enemies which helps your damage, but you could build the void staff to tear through tanks MR. (magic resistance=MR)

pros to fiddle is that his Fear can last for a seemingly long time allowing huge amounts of damage before they can turn on you. Your dark wind can be more useful when taking on closely grouped enemies as it silences and bounces between the enemies. cons to fiddle lie in his ultimate use. Being that it teleports you it has the potential to put you in a group of enemies who will take every chance (if they are smart) to kill you. His drain also has a relatively short cast range so you have to get close and for characters who have a dash like xin zhao and irelia running in for a drain could turn bad in an instant.