Just practice really. If you're doing a fight you're new to you're not likely to perform perfectly since you're still picking up intricacies of the fight. Eventually your combos and buff/debuff applications will kinda just fall to muscle memory and you won't have to think too much about them and can focus more on fight mechanics or maximizing your DPS. Dunno what class you're playing but if it's NIN then I had similar issues when I initially hit 50 despite having played MNK a ton in the past. Juggling Trick Attack and Kassatsu in to the rotations you've been using up til 50 just takes some getting used to (and familiarity with fights to maximize their use), there's no real shortcut to getting better at it other than just... playing the class some more until you get used to it.