Generally what I do is lookup particular job's "burst", and usually it's on Balance discord because they have a visual guide. I use it to have a general idea what to do, and AST is one of those that can get overwhelming at first because there's so much going on.

Depending on the party or raid, I always use the first melee card on melees (DRK if there's one, lvl 90+ only) and build up the seals from there. Since I only do casual content, min-maxing is not really that necessary. As @AmiableApkallu said, you really have to adapt to each situation, each healer is different (and so are people) and there's really no need to stress yourself out with being "perfect". I've had plenty of AST co-healers who never used Divination or cards, plenty of WHM who only used Medica and Cure 1, plenty of SCH/SGE who never used shields. Best way to practice is to throw yourself into the content and later analyze what you could've done better.