While I understand how to make the game overall, I'm kinda stumped on how I would go about using test sprites and backgrounds and what not. Like, if I wanted to test a specific room but only had the sketches done for everything. Any suggestions on how to go about this? I don't have the art fully implemented, but I want to start working on the rooms and stuff with the sketches I have just for a base placement, if that makes sense. Sorry if this is common knowledge! I'm stupid as hell, and couldn't really wrap my head around any way to do this. If you have any tips or ways you think I should go about this (in general) please let me know!
What part specifically confuses you?
You'd shrink down the sketch to the game's resolution and just use any sprite you have, if you have established sizes for sprites, make those sprites of those sizes (shrink/enlarge them). If them being a specific size doesn't matter, and you're using one of the templates that come with AGS, they usually already have some character sprites.
Basically what Babar said.
It's also quite common to use sketches and test sprites as placeholder artwork while you program your game, but before you've finished your artwork. That way you can just replace the artwork as needed.
Yeah, just to reiterate what the others have said, you can plug in any sprite or background as you wish and change/update them as you see fit.
Some devs like to have their whole game pretty much playable from start to finish with only mock-ups, sketches and stock photography, and then only start swapping out the placeholder art for the real artwork once they’re happy that all the gameplay elements are going to work. (This helps avoid wasting time on assets that aren’t going to end up in the game.)