My daughter-in-law wanted to put bright pillows on her outdoor furniture so we bought outdoor canvas that resists mold and mildew. I sometimes use it for indoor pillows because it holds its shape so well.
First of all you need the pillow form so that you can determine how much fabric to buy. For a 16 X 16 you will need 1/2 yard of fabric that is at least 40 inch wide. The front piece will be cut 17 inches by 17 inches, essentially adding 1/2 inch to each side to allow for the 1/2 inch seam. To make the overlapping back you will cut 2 rectangles 17 X 12. So for any size pillow form, the front panel will be the width plus 1 inch and length plus 1 inch. The back panels will be 3/4 of width and length plus 1 inch. If the fabric is lightweight, the back panel can be interfaced.
On the back panels, one of the long edges of each need to be finished as they will form the envelope. I use my Janome Coverpro because it will hem and overcast at the same time. I press under 1 inch on the length and then stitch on the front and the back is overcast.
Now you are ready to sew your pillow together. First lay down your front panel, right side up.
Next place 1 back panel wrong side up onto the front panel matching all three unfinished edges.
The next back panel is placed wrong side up, matching the opposite 3 unfinished edges.
Sew completely around the square backstitching at beginning and end. When you get to the section where the back sections overlap, backstitch over the 1 inch section to strengthen this area. Stop sewing 1/2 inch from end of side and pivot fabric to sew the next side.
Now turn right side out, use a dowel to push out the corner and press before inserting the pillow.