I made these covers out of a kind of fabric called "duck cloth". It is very thick - like canvas - because these covers get decorated by parents as a surprise for their children. So, we use thick solid colored fabric to make it easy to glue onto. Below is a picture of one of the parent's decorations:
These would also work great with cotton fabric!
1) Start by measuring the height of the book, and the width of the book. The width should including both covers and the spine.
2) Your seam allowance will partially depend on how thick your fabric is. My really thick fabric needed a 1.5" seam allowance on the width, and a 1" seam allowance on the height. My very first try made the whole thing just barely too short, and that was frustrating. I had only given myself 1 inch or for a seam allowance on the width.
3) Cut two pieces of fabric to the width x height + seam allowance dimensions.
Then, for the pockets, cut two pieces of fabric to the same height, but only 1/2 the width of one cover.
The picture below shows one of each of these pieces cut.
4) Fold one long edge of each pocket under by 1/4" and sew, to remove the raw edge.
5) Lay out your fabric by laying out one large piece, right side up. On top of that, lay your two pocket pieces, right side up, on the far left and far right edges.
7). Sew all the way around, with 1/4" seam allowance, leaving a 3" opening at the bottom middle to turn it right side out.
8) Turn the cover right side out. Iron it flat - making sure the opening is tucked in. (in my picture, mine is not ironed:) )
9) Sew around the entire cover, an 1/8" from the edge.
That's it! You are done. Slip the book into the pockets.