First of all, I have to apologize for the quality of some of these pictures. I didn't realize they weren't that great until after I was done. But, I think you can still get the idea of what I was trying to portray.
I started with this small binder I have had forever, but you could use any size notebook and adjust the measurements accordingly.
1) Measure the notebook you want to cover (mine is about 5 1/4 inches by 8 1/4 inches).
2) Cut a large rectangle of your outer fabric that is large enough to accommodate a 1/4 seam allowance on each side plus 1/8-1/4 inch around for the top stitching. (I failed to take the top stitching into account in my measurements so mine is pretty snug. I will provide example measurements of what I should have done.) For my notebook I needed 6 inches by 9 inches. If you aren't sure, I would recommend going a little larger rather than a little smaller since you really can't fit the notebook in there if it is too small.
3) Cut 2 small rectangles of coordinating fabric for the inner pockets. The size of the pocket depends on what you like and the size of your notebook. I wanted mine to cover about 2/3 of the notebook. These need to be the same measurement up and down as the other rectangle you just cut ( in my case 6 inches). Decide on the final width you want and add at least 7/8 inches for seam allowance, top stitching and the hem on the open side. I wanted mine to be 2 1/2 inches wide so each rectangle ended up 6 by 3 3/8 inches.
4) Cut a strip of coordinating fabric for the closure. Again, the size of this depends on the size of your notebook and your preferences, but I made mine about 1 3/4 inches wide and about 5 3/4 long. It was actually just a scrap that was left over from my other cutting and it looked about right so I used it.
5) Cut fusible interfacing the same size as your main fabric and the rectangles for the inner pockets and iron on (wrong sides together) as the package instructs. I used lightweight fusible interfacing. This is optional, but I think it helps give it just a little stiffness and form and also helps the lighter areas of the fabric from being see-through.
6) Iron a 1/4 inch double hem along the side that will be the open side of each of the inner pockets and sew.
7) Fold the strip that will be the closure in half, right sides together, and sew along the long side (1/4 inch seam). Then turn right side out and iron with the seam in the middle.
8) Position the closure strip onto the main fabric (right sides together). It will be on the part that will be the back of the notebook.
9) Layer the pocket piece on top of the outer fabric and closure (again the right side of the pocket should be facing the right side of the outer fabric and the hem should be towards the middle of the rectangle) and pin into place. Also pin the other pocket piece into place.
10) Stitch around each pocket piece from where the hemmed edge meets the outer fabric all the way around to the the other hemmed edge of the same pocket. Do this on each side and then turn right side out.
11) Fold in and iron the piece of outer fabric between the 2 pockets on top and bottom.
12) Top stitch (1/8) inch around the entire notebook cover.
13) Slip the notebook into the cover and determine where the button should be positioned and hand-stitch it on.