I started with a block pattern I found on Pinterest. Pattern available at Mango Tree Crochet. I found this lovely cotton yarn while visiting Edinburgh, Scotland, in a cute little shop along The Shambles. I used a G hook. My finished square turned out 17 inches.
I added 2 rows of my own to make the pattern into a square. You can follow the directions below.
sc -- single crochet
hdc --half double crochet
dc -- double crochet
Row 1: Start with a standing sc in the middle stitch of the chain. dc in the next 3 stitches. hdc in the next 3 stitches. (2 dc, ch 3, 2 dc) in the corner. [hdc in the next 3, dc in the next 3, sc in the top of the chain, dc in the next 3, hdc in the next 3, (2 dc, ch3, 2 dc) in the corner,] repeat from  3 times. hdc in next 3, dc in next 3. Slip stitch to the starting sc.
Row 2: ch 3. dc in each stitch around with (2 dc, ch3, 2 dc) in each corner. Slip stitch to starting ch and tie off.
Squares are stitched together with sc.
After blocking the crocheted square, I measured out the backing fabric and made a slip cover style pillow case. Mine has a snap closure in the back so the filler insert can be removed. I then hand-stitched the crocheted block to the pillow case.