I think pencil cases you used at your school time were bought from the stationery store. At least I didn't DIY pencil case when I was at school. But now I will give a pencil case tutorial for you.
The pencil cases I had used are almost made of plastic with a lid to open and close. Now many kinds of pencil cases are coming out. This pencil case tutorial will show one kind of felt one. Well, let's go to the point of DIY pencil case.