The basic cushion case size is about 18 inch x 18 inch laid flat (it's much easier to use a old cushion cover as a template than to use the cushion itself)
What you'll need
18 x 18 inch of pattern fabric
18 x 10 inch of plain fabric (I used a natural twill)
18 x 14 inch of plain fabric
If you don't have enough fabric you can do 2 x 18 x 10.5 inch and use two metal snaps or buttons (I did the first lot this way but decided I like the idea of a proper envelope slip case)
This is the basic idea of how the overlapped fabric works
What to do
Grab the 18 x 10 inch piece of fabric
Fold the 18 inch side down one inch + iron
Fold the 18 inch side down 1/2 inch so it aligns with the crease just ironed + iron again
With the raw edge of the 18 inch side folded in; iron again
Fold it over itself again
Flip over and stitch down the hem
Do this with the other backing piece of plain fabric as well
Place the patterned fabric face up.
Align the edges of the short rectangle with the patterned fabric and pin.
Align the edges of the large rectangle with the patterned fabric and pin.
At the overlap it should look like this with the short rectangle tucked under the large one
You can stitch around with a straight stitch and then hem the raw edges with a zigzag stitch or I found the opposite way to be easier. By hemming the raw edge first the pieces held together perfectly and when it came to doing the straight stitch I could just zip straight around.
And that's basically it, the quick way to make cushion covers. You could add a zip if you like or studs if you don't have enough fabric but this is a great beginner's project.