There's a bunch of very general "what to do at the start of a season" guides on reddit, both overall and specifically for classes in the classes subreddits. It's not really that complicated, a lot of it is just "you farm until you get items that makes what you're doing too easy and increase difficulty". There's a few different things to consider, like doing bounties for materials to cube powers and doing GRs to get legendary gems and level them and doing ubers to get hellfire, but ultimately a large part of the early progression is literally, open a rift, do the rift, close the rift, rinse, repeat and the rest can be ignored entirely until you have 6pc of your set. I mean, I'd probably mix activities a bit since it's a faster progression(even lvl 1 legendary gems and 3 random powers are better than nothing) and quite a bit less boring, but you wouldn't be wrong. It's still Diablo and a lot of it is still just mindlessly killing a lot of shit to get some more powerful items.
After that, following high end builds on Diablofans works well enough, it's a guideline for gearing purposes to follow, while you can adapt stuff to your own difficulty before you assemble all the items. Tweaking your stuff yourself is the best part imo, it starts sucking when you have to do the cookie cutter shit to progress cause it's the only decent thing, before you get there when you're using weird legendaries and skills because they're more fun and you don't need nearly as much damage/toughness to do your difficulty, it's a lot more fun.
Crusader is poop though, I hope they manage to rework the fucking class before they release the next expansion. At some point they need to stop for a bit and just redo everything so the class stops being so bad. They've been improving it over the patches bit by bit, making some passives less shit and some skills less shit(falling sword for example in this patch), but there's still so many wrong things with the class, at a basic level, it's really sad. Doesn't help that they can't balance their sets for shit either, so on top of having a shit base level, they also have a shit top end. Fucking top crusaders solo GR isn't even top 500 monk GR I think, that's so fucking sad.