How can I free up space on my C drive or combine it with my D drive?

I'm dealing with a full C drive—only about 4.69 GB left—and my D drive has plenty of space (919 GB). I tried to combine the two drives but ran into issues. Whenever I attempt to clear up space on C, it looks like I'm just deleting system files, which I know I shouldn't do. I've also tried deleting temporary files using %temp%, but that hasn't helped either. Does anyone have advice on how to either combine these drives or effectively free up space on my C drive? Thanks a lot!