The square begins with CO 3 sts. Every RS rows increase 1 stitch on each side. As a border used 3-stitches garter pattern, the background is stockinette stitch pattern. When measurement will be 12” on the bias side begin decreases - 1 stitch on each side every other rows.
The pattern available in English and French. Special thanks to
Ysabeau Caradjine for translating the pattern to French!