This is obviously going to be very different for each game but I'm wondering if there's any good resources out there for ways to indicate to the player how to play - every time I make something I feel like I'm reinventing the wheel (and it seems like most tutorials in general completely skip this step). For my current project, I'm going at it where I draw a black rectangle with low alpha across the whole screen and then highlight things with textboxes (but it's honestly pretty laborious and the code feels clunky).
Textboxes? That's already the worst type of tutorial, nobody likes
reading 
The best tutorial would put the player in a situation where:
- the only way to proceed is to do the thing you want to teach them,
- the thing they're meant to do is obvious,
- there's no risk of failure
For instance, to teach them to jump, place a ledge in front of them that's so tall, the only way to get on top of it is to jump. To make it extra clear they're meant to go up there, place some coins / gems / apples there. To teach them to crawl, make the only way forward be a narrow passage but put some collectibles inside it so they can tell they're meant to go there. (Both of these examples fulfill both the "only way forward" and the "obvious" points above).
The "no risk of failure" point is important for several reasons, including that players will stop experimenting if they realize they can lose something important if they mess up... and they might not even try the thing they're supposed to learn if that happens, leading to frustration. (Same thing if they repeatedly try the
wrong thing, and die and need to redo a lot of content as a result). If the thing you're trying to teach them absolutely
must have a risk of failure attached to it (e.g. you're teaching them about lava), placing it early in a level / close to a checkpoint / showing it off by having an NPC/enemy die to it before the player gets a chance can help soften the blow.
For complicated interactions, many games also have approaches where the player will automatically land on a trigger to set off an effect after a screen transition, so they will see it in action without actually
intending to trigger it (e.g. Shovel Knight teaching you about the green slime cooling down lava in Mole Knight's stage)
There's a number of other things that are good to keep in mind:
- Have several challenges of escalating difficulty whenever you teach something new (e.g. first crawl through a featureless tunnel, then emerge from a tunnel into turret fire but since you're crawling you automatically dodge it, and finally face a turret WITHOUT a tunnel but figure out that you can dodge it by crawling even when there's nothing to crawl under) so the player gets eased into using the new thing you taught them.
- Only use text when absolutely necessary (telling the player something that can't be intuited from using a mechanic, like a new button combination). Seeing something happen works in every language, reading an explanation does not.
- Only introduce mechanics when you actually need them, or the player will forget about them. Spread out the tutorialization moments over the entire game and have gimmick levels that introduce new stuff.
- Reward the player for using a newly-introduced mechanic, even if it's just with meaningless points. Having a bunch of (poorly) hidden areas with goodies for the players that did their homework will encourage them to use the mechanic whenever possible.
