
I'm not too sure about this tutorial. Most of you could whip the pillow up in a jiffy just by looking at it. I'm still fiddling with the idea and have not yet decided the best solution for the page holders. Oh, well, here it goes:
Measure your pillow and add 3/4' to each side. The pillows I bought were 12 x 16, and for this pillow, I cut 2 pieces 12 3/4' x 16 3/4".
Heavier, home dec fabric works much better than regular fabric as the pillow needs some stiffness. (none of the pictures show the color accurately)
I've done the back two different ways, but this is the simplest. (See the bottom of this post for a two-piece back.)
Sooooo...you have 2 pieces the same size for front and back, and
a piece the same length as the pill0w and 6" deep for the pocket.
Turn under 1/4" along the top of the pocket piece and press; turn under 1/4" again and sew to finish top edge of pocket
Pin the pocket to the front piece and pin.
Mark the pocket with chalk or water-soluble pen. I have made 3 pockets, a small one on each side, a larger one in the middle on mine. Two lines of stitching.

Sew the pocket from the bottom to the top of the pocket and back stitch to make it secure.
I've done the page holders differently on every pillow. You can see the variations at the bottom of this post.
For this pillow , I decided to start at the bottom instead of the top and to use some rat tail that I already had (no elastic on this one). The rat tail was too bright, so it had to have a strong coffee soak to dull it down. Experiment with how long the ribbon/rat tail should be to hold your book securely, including enough to wrap around the buttons.
Pin the ribbon/rat tail to the center of the right side, at the bottom of the pocket.

Pin the pillow back to the front, right sides together (you can tuck the ribbon/rat tail in one of the pockets to keep the loose ends out of the way). Leave a large gap at the top to stuff the pillow in!
Sew. I used a 1/2" seam on this one (but I found the pillow sizes varied slightly...maybe because of the stuffing or brand). I know that isn't what the picture shows, because I just put the fabric under the foot and snapped the shot. Should have paid more attention.
Turn the pillow inside out to see how the pillow fits. I've had to go back and make a bigger seam on some pillows. One pillow was just too puffy, and I took some of the stuffing out. When the fit is satisfactory, trim the corners to reduce bulk, turn again, and stuff the pillow in, pin and slip stitch the insertion opening.
On this one, I covered some buttons with the same material and sewed them to the top of the pillow. On another, I used vintage buttons.
When you're ready to read, place your book on the pillow and wrap the rat tail around the buttons to secure the pages, put a book mark, high lighter, pen, or whatever in the pockets,and its good to go.
I hope this makes sufficient sense. Such an easy process, but explaining it seems iffy.
Page holder variations:
covered 1/4" elastic version
elastic band version
grosgrain ribbon and buttons at bottom version
I have done the solid back on two pillows and the envelope back on two.
Here is the envelope back being pinned to the front. Instead of one back piece, I cut two pieces long enough to overlap and finished the edges and then pinned it on. You can insert and remove the pillow much more easily. 
I have a couple of other things I want to try for holding the pages, but I'm a little tired of the darn things right now.
If there is anything I can clarify, please let me know, and I'll try.