Let's say your quilt is being made out of 6" blocks.
Take a 6 1/2" square as your back ground fabric.
Take another 6 1/2" square out of the same or complimentary fabric
fold it in half, right sides out, press a good crease into it.
Lay it on the first square, the folded crease going across the center and the raw edges going along the sides and bottom.
stitch 1/8" around the folded fabric to hold it in place.
Add this block around the quilt to tuck your notes into.
You can adjust the height of the pocket by cutting the length of the block longer or shorter.
You can also make one out of three squares of fabric. Fold 2 of the squares into a triangle, press a crease into them.
Lay one across the lower corner raw edges to the left and the bottom with the crease going at a 45 degree angle across the main block. Lay the other piece with the crease going across the opposite way. Stitch it down the sides of the folded pieces with an 1/8" seam. Where the fabrics cross will also give you a decorative pocket.
If you want it to look like an envelope, add this to the one above. Cut an additional 7 1/2 or 8 inch square, fold it into a triangle, right sides together, sew around the outsides, leaving a section unsewn so you can turn it out. hand stitch the opening closed. Place this piece at the top of the block with the point down and covering the opening of the pocket. Trim it to fit the block and sew the top in place with an 1/8" seam. Sew on a small piece of velcro to the flap and envelope, to hold the flap in place.
Try it with paper first to get the look you are after :wink: