The logic of how to make a simple diablo style inventory system?

Hi I'm attempting to make a horror game in gamemaker. Really just to see if I can and maybe to get some freinds to play it. And this is the first "complex" game I'm coding, and I'm surely a better artist than coder but progress is pretty good, Thanks to the abundance of tutorials out there. except for just this one thing and it's really telling that I'm getting stuck the moment I can't find a clear tutorial to follow cough steal and ducktape to my game cough

What i mean by diablo style inventory is essentially the inventory tetris you know what i mean, like in diablo. I've found only one tutorial for how to do it on yt but I'm 90 percent sure I have some kind of learning disability or adhd and I can't for the life of me follow along with the video. And I don't want to stoop lower than dirt and ask chatGTP so I came here. If you can give anything I just want the logic of what I need to do. I've already made structs for items, and made an array to store the items the player picks up. And I've tried yo use ds_grids and I'm probably doing something wrong but it keeps never working and it's getting frustrating.