Not Updated For Current Season
Early Game : Start with an amplifying Tome and a potion (HP or MP) and lane with a mana user champion to maximize the use of clarity. Return to base when you have 750 gold to buy the sorcerer's shoes. (If you have a hard time finishing your enemy return to base at 350 gold to buy boots of speed.)
Mid-Game : It's time to turn your Amplifying Tome into Hextech Revolver and start farming the jungle/empty lane to get your Guinsoo's Rageblade fast.(You might want to buy a Blasting Wand before the blade itself since the blade cost is 2235 gold.)
Late Game : First of all, turn your Hextech Revolver into the Hextech Gunblade. (Don't forget this item have a Active skill.) At this point it really depend on who you are fighting against. Nashor's Tooth, Lich Bane and Trinity .. in any order will do good.
If you want me to write more on this build just ask. :)