Here is how I made it:
First I measured my pillow and cut out two rectangles out of my gray broadcloth adding a 1/2 inch seam allowance on all sides and an extra inch on the back piece for the zipper.
Next I decided where I wanted my zipper to go (I chose about 1/3 from the top) and cut a straight line across the back piece of broadcloth.
Next I attached the zipper using this tutorial.
Now I set aside the back piece and focused on the front. First I cut out about 1 1/2 yards worth of circles out of my felt (it takes a lot more than you might think). I used the top of a glass for a stencil - mine was about 3 inches in diameter, but yours can be whatever size you want.
Next I folded each circle in half and pressed it to make them stand up and to give a stitching line.
I started randomly placing the circles on the front of my broadcloth and sewing each one in the ditch of the pressed crease. I left about an inch around the edges to assure that I could stitch the front and back together.
Once the pillow front was looking pretty full I stitched the top and one side together at a 1/2 inch seam allowance. I pinned the circles away from the seam allowance first so that I wouldn't get them caught in the stitching (don't worry if you get small bit caught in - it's inevitable and not noticable once the pillow is finished).
Now I can go back and fill in the edges, allowing the circles to overlap the seam line a bit.
Turning it inside out again, I stitched the remaining two edges together and then flipped it right side out through the zipper. Now I was able to fill in the edges of these last two sides.
Lastly, I clipped the many loose strings off, put my thrifted pillow inside, and I was done.
I couldn't be more happy with how this turned out.