here's what you need to make a cover for a ring notebook:
Felt the color you want (I did it white)
The first thing to do is take measurements of the notebook.
cut out the felt of the same measures but leave 1 cm of space.
cut two strips of equal height and length as you want.
sew the strips to the large rectangle of felt.
If you want you can add decorations by sewing them to the felt.
folded inside the sewn edges.