This is a huge thing right here that so many people overlook.
Practice on the freaking target dummies. They are there for a reason.
My friend/guild group is…mostly not great. We have a few mid to high key runners, but the majority are not capable of super high content and we have to carry them quite a bit. They are constantly asking us what they’re doing wrong or how to get better.
The first thing I always tell them is that I spend a copious amount of time on the target dummies. It’s the first thing I do to warm up when I sit down to play, before I go running a bunch of keys. It’s the first thing I do when I have a new build, or a new spec entirely. It’s the first thing I do after vault or when I get new gear/enchants. Etc.
Hell, I hammer on the target dummies out of pure boredom at times.
Gear, guides, a fundamental understanding of the spec and the talents and all that is important…but getting the muscle memory down and having it be instinct/second nature (not to mention getting a good idea of where you’re at and if you need to improve) is purely done through experience. And I would rather that experience be fundamentally rooted by target dummy practice rather than by fumbling keys and wasting everyone’s time and emotional battery.
Then your mental capacity is freed up a bit to focus more on CC, ints, dispels/purges, mechanics, etc. and you continue to grow.