12" x 16" pillow form (sold at JoAnn's and other craft stores)
1/4 yard "main print" fabric (or at least 8 1/2" x 44")
Do this for each of your three sets of pieces and press the seams open:
At this point, You should have three pieces:
I like to use a zig-zag stitch to sew the hem:
You are ready to line the two smaller pieces on top of the 13 1/2" x 17 1/2" piece, placing right sides together. The two hemmed edges will overlap, becoming your envelope flap to insert the pillow form.
Once you get your pieces "sandwiched" and lined up, it's a good idea to pin it all in place. (The two pieces on top tend to slide around.) Sew a 1/4" seam around all four sides, reinforcing your corners with some back stitches.
Remove the pins and turn your pillow cover right side out.
This is a great travel pillow for adults too :)