For me, I started playing the class and learn about my cast sequence habits.

I also label how many button presses I need to complete the sequence.

Shadow Priest:
8 = Vamp Em, Vamp T, SWP. MB, MF, MF, MF, Shoot
5 = Vamp Em, SWP. MB, MF, Shoot
4 = SWP. MB, MF, Shoot

etc.

Eventually you will add "pauses" for each keypress to allow for casting time.


Now do the same for your Mage or Warlock.
Each one will have it's own cast sequence, and you will see similar key presses and pauses.
Bind those up and you have 1 button to press in your main, and your dps will do it at the same time.