Also would I need to use interfacing to strengthen the top button hole side?
Firstly apologies in the length of time it has taken to reply.
The sides can overlap about 2-3 inches depending on the size of your buttons. This should allow the fabric to lie flat without gaping. If you wanted more of an overlap then that would be fine as sewing is really trying and adjusting if necessary. The addition of interfacing would strengthen the buttonhole side and could be used on the button side as well so the fabric doesn't looked pulled when they are fastened.
Hope your cushions have turned out well.
If you have found an error on this page or want some more information on sewing cushions.
Please contact me . Thank you