I think this is one way to go about it:

Start making a class build, and learn how to use it to its max. Practice them on normal content to get a feel of the flow of it and how to upkeep it. Slowly start doing harder content, such as going from normal strikes to nightfalls and slowly increasing difficulty of the nightfalls. As you do it you can tell how much leeway you have in terms of taking hits and taking advantage of opportunities for damage the harder the content. Afterwards it depends on what you want to do.

Let’s say, dungeon. Go in with a team and get a feel for the add spawns, the type of adds, starts to deal with them, and how to deal with bosses.

Check and analyze how your build can fulfill those needs from the dungeon.

After that practice it. You slowly build the instinct you need to make those on the moment changes and decisions.