I colored in a 45 degree diagonal line with some black permanent marker on the wood first then folded the material from each edge at a 45 degree angle, butted them together, then stapled them to the wood from behind.
The black permanent marker was just in case there is any slight separation of the miter at some future point. You won't notice it.
EDIT: I re-did the miter permanently eliminating the possibility of a space.
On the two short edges (left and right) I simply continued up and over the top and bottom stapled it to the back - NO 45 degree angle.
Then I folded a 45 degree angle on the ends of the top and bottom pieces of material and overlapped the edge pieces (in other words a mitered joint with only ONE 45 degree angle). It looks just like a true mitered edge only much neater and no chance of a space because it is overlapped.