Interface
The important part is having a good weak aura setup for your class/spec and a good nameplate setup. But more than not, UI’s are personal preference. However, you should look into keeping the center around your character quite empty, so you can see mechanics around your character.
The most important addons you will need is Weakauras and a nameplate addon (I recommend Plater).
Macros
Deathmark + Trinket + Potion Macro
#showtooltip Deathmark
/use Tempered Potion
/use 13
/use 14
/cast Deathmark
Additionally you can add your on-use racial into this macro as well. As an Orc on the beta realm, I added:
/cast Blood Fury
Easy Mode Cold Blood
#showtooltip Cold Blood
/cast Cold Blood
/cast Kingsbane
This macro is actually optimal, even though Cold Blood does not work with Kingsbane. Macroing it in this manner is purely for the timing to line up with Edge Case flips during Deathmark
Shiv + Tea Macro
#showtooltip Shiv
/cast Shiv
/cast Thistle Tea
This macro completely removes Thistle Tea as a button from the spec, and is optimal. After the 11.0.5 changes we no longer focus on the energy from Thistle Tea, and instead we cast it with Shiv for the mastery during our cooldowns.
Focus/Mouseover Macros
/cast [@focus] XXX
These can be used for any CC, including Kick, Blind, Kidney Shot, and help keep important mobs in check. There’s a variant I’m fond of which automatically also makes the target you’ve kicked your focus:
#showtooltip Kick
/cast [@focus] Kick
/focus [harm]
The way this macro works is that you press it once without to focus a target. Upon having a focus, any press will kick that target. You can also add modifier conditions to this if that helps.
Weakauras
My Weakaura Package can be found here: https://wago.io/a4RNXiGN8
My Plater can be found here: https://wago.io/t6bI_B9wi
Lisonna is a Weakaura savant who has her own package, as well as many standalone auras for rogues, you can find her work on her profile here: https://wago.io/TyRLPd41N
Realistically any weakaura or plater package will do as lot as it fits your eye visually and removes some of the noise in AoE situations. Assassination has many buffs and debuffs, a lot of which aren’t necessary at all. Filtering out this clutter can be a great help for many rogues.