The final step of these cushion is definitely the easiest, and also probably the most useful because you can use it to make any simple cushion (you can find all the other parts here). In fact all the cushions in the above photo are made in this way. So you can make some to match (or not match like mine) to go with your fancy patchwork, ooh la la.
To finish off the cushions I used more white fabric but you can go with anything you like, more patchwork, the same fabric as the front, or go crazy (well within reason) and use something totally different.
You need to 'hem' one of the longer edges of each rectangle. Hemming is a seriously useful technique and you will use it again and again. To hem the long edge, turn your iron on to the hottest heat your fabric can handle, test it first on a bit you won't be using. Then fold over about 1cm, towards the wrong side of your fabric, of this long edge and press it down with you hot iron, then fold it over again 1cm and press again. Pin this down and then sew along the edge of the folds on both backing rectangles:
Finally turn your cushion cover out the right way through the hole and press the edges. Stuff it with your cushion par and Tada! You make a cushion!