Interface
Interfaces are a lot of personal preference, so finding something that works for you may not be what works for someone else.
For me, I like to use ELVUI; the bars and unit frames are exactly what I’m used to, and it’s very easy to set up and maintain.
I use Plater for nameplates; it is not quite as customisable as it once was, but it’s very important to have good nameplates for applying DoTs to multiple targets.
For the cooldown manager, I’m currently using Arc UI; so far, it is the best solution I have found to modify the cooldown manager.
Macros
Mouseover macros for applying DoTs.
Agony Macro
Casts Agony on your mouseover or your target if there is nothing under your cursor.
#showtooltip Agony
/cast [@mouseover,harm][exists] Agony
Corruption Macro
Casts Corruption on your mouseover or your target if there is nothing under your cursor.
#showtooltip Agony
/cast [@mouseover,harm][exists] Corruption
Demonic Gates Macro
Casts instantly on your cursor without the need to double click.
#showtooltip
/cast [@cursor] Demonic Gates
Healthstone + Soulburn Macro
Automatically casts Soulburn before your Healthstone, very important.
#showtooltip Demonic Healthstone
/cast soulburn
/use Demonic Healthstone
Focus Interrupt Macro
Can be very useful in M+ or Raid, where you don't want to miss an important interrupt assignment.
/cast [@focus] Spell Lock