Beginner trying to figure out extract

Hey there.

So, I’m working on a tutorial from Udemy where I’m learning to make a treasure chest.

Basically, where I’m hitting a wall is when it comes extruding something inwards. I’ve attacheda pic of the desired outcome from the tutorial. I can’t actually get the part of the faces that I’m trying to take inwards any further than where it is.

Thanks!
Matt!