Basically, DO THE THIEVES GUILD STORYLINE BEFORE BEING AN ASSASSIN.
You would not believe how awesome it is to use Shadowcloack of Nocturnal, turn invisible, silent rolling your way to an elevated position above your targent, taking your bow out, killing your target in one hit, silent rolling away in less than a minute and do a "Perfect Assassination".
Ofcourse, if your the kind who just wants to kill everyone with his blade, ignore this.
Garry Damrau wrote: Or you could just use invisibility potions to accomplish the same result.
Or the Shadow Stone, or vampiric powers, or maxed-out Stealth. Or, if you have all of them, you can just walk up to your target, stab them once for 15X damage, then turn invisible again, and sneak away.