Use needles & hook according to yarn used. I used oddments of double knitting yarn.

The central square is knitted, then single crochets around the square, (3 single crochets in each corner on all rounds).

The cushion is diagonally folded into 4 and seams sewn. The seams are straight stitched together and are on the outside. Stitch 3 seams, push the cushion in, then sew up the final seam.
That’s about it :)