Bracer: it's very important because SS he has a very short life.
Boots of Travel: Someone prefer Arcane Boots, because SS needs a lot of mana, but I think that the best use of this hero is like a pusher/tower destroyer, and the best way to do this is by buying Boots of Travel, that allows you to push very well and, with some skill,
to escape in critical moments.
Oblivion Stuff: it gives you a good intelligence and a good mana regen, and it will be used in the Refresher Orb.
Aghanim's Scepter: it's a must have for SS, because makes your ulti more powerfull.
Refresher Orb: RO allows you to use your ulti two times, this is very goood for team fight and for destroying enemy'tower.
Bloodstone: it's great because give you a lot of mana regen and hp and if you die you can respawn more quickly.
With this skill build, you will able to kill from level 6.
Imho the best way to kill is use Hex, Mass Serpent Ward and Shackles(with this sequence).
If the enemy hero has a lot of life, you may need to use Ether Shock while Shackles ends to give him the last hit.