Fold the least favourite tea-towel in half as pictured and chop. This will be the back of the cushion.

Take the other tea-towel and fold one corner up to make a square as pictured and chop. Keep the left over bit for trim or other projects.

With the square bit right side up, place one of the back pieces face down on top of it as pictured. This bit will end up as the 'showing' part of the back.

Place the other back part face down on top of the other two parts so that its hemmed edge overlaps that of the first part. Then pin all the way around the edges.

Sew straight stitch all the way around the edge of the square and remove pins.

Snip off the excess fabric at the corners then turn the cushion cover right sides out.

Squish your cushion in through the back and voilà! Told you it was ridiculously easy.
The back should have enough overlap that you don't have to do anything further to the cushion unless you want to, but you can add ribbon, buttons or toggles to hold the back flap shut, or trim the front with something decorative. I generally find after the 6 minutes this has taken me I need a bit of a rest so don't usually bother ;-)

The finished cushion!