Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
As for starter items, I go for Doran's Shield and a Health Potion.
Then I get Ruby Crystal, Boots of Speed and Sapphire Crystal if possible.
Upgrade to Catalyst the Protector.
Finish to Ninja Tabi.
Build Mejai's Soulstealer.
Finish Banchee's Veil.
Build Rylai's Crystal Scepter.
I didn't add a 6th item in the normal build, because it heavily depends on how the game is going and who your enemies are.
I'd either go for Zhonya's, Nature's Will or Guardian Angel.
I just keep cool in early game, cause ur not likely to make any kills the first few lvls but try to make as many last hits as possible.
From lvl 4 on you are more dangerous, because ull have ur 3 skills now. If ur enemies are getting a little too aggressive, throw a Mega Adhesive slow just behind them if needed and Fling them so ur laningpartner can do some dmg as well and you might be able to make a kill out of it with ghost and ignite.
Dont worry if you didnt manage to make a kill before lvl 6, because singed isnt such a great early game champion.
From lvl 6 on you can start to do some real damage to people and it gets pretty easy to chase or even towerdive with Insanity Potion + Ghost.
Help the team as much as possible and join many fights for assists because you want stacks on ur Leviathan and Mejai's Soulstealer.
In teamfights Fling the enemy carry into danger or if someone is trying to kill ur ally carry, Fling the enemy away.