for university.. who said I was grown up? oO
So, you can find a How-To for this on another website, it's a german sewing page with loads of tutorials. As an english-speaking person, it might be hard for you to understand it, but there are pictures and I'm gonna give a short description about every step. In fact, it's not that hard at all.
Lay your folder on the fabric. In the Length, the fabric should be twice as long as the folder.
In the breadth, as broad as the folder is (opened!), don't forget seam allowance.
Cut out the marked fabric and pin it together as shown on the picture in the link (I wasn't sure if it was allowed to upload those pics from the How-to). Cut holes for the ... metal... middle..part.. (I don't even know a german word for it :D)
Afterwards, remove the folder from the fabric and cut a bigger hole and make the shape nicer etc.
Attention! The original How To was meant to be for a cover that can be removed from the folder (for washing etc), I ignored that and don't have the option to remove my folder anymore throughout the hole. (Humm...whatever..)
So, if you want a cover thats removable, cut a big hole and test if you can pull the folder through the hole.
If not, just leave the hole tiny and sew some ribbon around it. :)
Sew the pinned lines together :) Turn inside out, put in the folder and then you're done :)
You may add extras like loops for pencils etc.
I'm gonna make a pencil case from this lovely fabric, so I don't need that ;)