First, take Ambush and Garrote off of your action bar or move them to positions awkward enough so that you don't use them in PVE. Make macros instead of just using spells that do /attack on /cast mutliate and /attack on /cast envenom and use those. Add the finishers to your stealth bar, as well. My mutilate and envenom are in the same spot on the stealth bar and the normal bar.
They will look something like this:
#showtooltip Mutilate (rank)
/attack on
/cast Mutilate (rank)
#showtooltip Envenom (rank)
/attack on
/cast Envenom (rank)
Shift+click the moves from your spell book (P) instead of typing the names of the moves out, and it will include the rank automatically. Of course, my macros have trinket clicks and cancel auras for HOP in there, as well, but that's a little more advanced.
Open from stealth with mutilate > snd > hfb > mutilate > envenom. When you vanish, don't pool energy unless the envenom buff was coincidentally up long enough for you to have the energy pooled when you vanish. I normally do the first vanish as soon as the first overkill wears off and then on cool-down, but I particularly watch for it to be available if I don't get a combo point proc from Ruthlessness because the next envenom is going to require two mutilates. I use mutilate or envenom to break out of vanish (with the macros mentioned above) without breaking cycles so it's pretty seamless.
There are also fights where you want to save your vanish like Putricide and Sindragosa, so keep that in mind.