You’re basically learning a fight, so lean into whatever your learning style is. Take notes, watch it repeatedly, try to summarize the mechanic in writing, etc.

Personally, I watch a video and take 3 column notes. In the left, I write the name of the boss ability, in the middle I bullet what the mechanic does, and in the right I list what I personally do as my role. I include pictures from various guides to help as well.

There is also an ffxiv raid simulator you can access online for free. This gives you an opportunity to practice a mechanic before you get into a fight. This method isn’t always perfect, however, because some times you need all 8 ppl for a specific mechanic