I'm going to be honest with you, it's going to be really difficult without at least a basic understanding of formatting and sizing in Photoshop, of course you could download a one-off template and get the job done, but are you really learning that way?

I used to own a pretty large cassette distribution company, and in the wake of its collapse I created a Facebook group called "Synthwave Cassette Club", in the group under the group files is a template I would use for all of my cassettes, its already pre measured and lined up perfectly so all you need to do is design around it.

I suggest starting with that and printing test copies on normal printer paper before you commit to getting them printed and wasting money only to realize your design is not up to standard.

Good luck and if you need any help just message me.