Go download Hekili, import your SimC data and follow the spell buttons exactly as they tell you to.
Than when you get used to the rotation your can start to adjust what Hekili tells you and what buttons you press. For example with an Unholy DK, Hekili will tell you to hit Death Coil when you have enough power. But you may be low on health and since Death Strike cost the same you could use that as a situational option instead of opting for max DPS.
For a BM Hunter Hekili may tell you to press Cobra Shot, but your Barbed Shot may only have a low time left before it expires and you might find it better to wait a second, hit Barbed Shot and than Cobra Shot so you don’t run out of the buff.
See Hekili is great against say a training dummy or a boss that just stands there, it has options to use defenses as needed, but it does not take anything into account as to what is happening, including lag, it solely runs whatever you input into it, therefore if you are inputting your SimC data and don’t turn on the defensive options it will 100% be tuned for DPS.
But if you stand there in front of a dummy or run some LFG it will teach you a better rotation to try and maximise your DPS if all works well and once you are more used to it, than you can adjust Hekili or just get used to what buttons show up and know what it means, rather than what to press.
That is, when Death Coil comes up it means I have 45 Runic Power, it wants me to press that to maximise my DPS, but I can choose now what to do with that 45 Runic Power if I need something else done.
Mind you, this addon does not work for some classes unless you have it set to single target, like Warlocks and Shadow Priests since they need to multi dot and the addon not only wants you to dot everything, but it wants you to dot it twice to maximise your time the spell is up on the enemy, which is not always needed for AOE.
I find most of the time I am around 10% less than the sim depending on the class, my hunter can hit closer to 5% lower, my priest could be as low as 20% less DPS than the sims.
The hardest part of being DPS is finding the right class that suits you.
You need the right gear, the optimal rotation, learn how to change the rotation based on what is happening, maybe choose to use something else. Like a boss may have 2% health left and there are adds, it’s stupid to use AOE as you want to maximise the boss damage.
It’s actually not easy being a DPS at the highest levels, especially when you start to sim your DPS. I’ve seen a guy who just couldn’t get it right at all, he had a good number of our top players in guild guiding him, he tried all number of addons, no addons, different talents, gear, you name it, he was hitting some 20K less than the other same class players with a similar iLVL.
You really need to be tuned to your class to be able to hit at mythic raid levels.