On zips thanks to the buttons! super easy and great for using old materials :o)
No backing on this but you could add if your fabric is thin.
Make a template allowing for 1-2 cm seams all the way round. Tack to the fabric and then cut out. Keep the buttons fastend when cutting.
I measured the cushion after buying (I have another shirt to do which is bigger) thankfully this was big enough (keep in mind is buying - I was lucky ha ha!)