I started by washing and ironing the shirt. I'm really not a fan of ironing shirts - as you can tell from the photo!
I picked up a selection of (free!) cushion inserts from Freecycle and used the one which was 20x20 inches, so cut a square from the shirt of 21x21 inches (to allow for a 1/2 inch hem). I wanted to keep the pocket, which meant a bit of fiddling around trying to get the arm hole seams to match up, front and back.
The original buttons were a bit beige, so I swapped them for some red ones. I think I must have been subconsciously thinking of Christmas when I picked the green and red colours!
The beauty of making this type of cover is that you can take it off for washing, but you don't need to worry about adding a zip as you have a ready made closing!
What have you made into a cushion cover?!
Update 15th Oct 14 - This post was featured here at Wake Up Wednesday