I think the easiest way to describe this is an envelope shape, pillow case with flap and button fastening. Grab you're tools, fabric, sewing machine (definitely achievable by hand just will take longer) needle and thread, button(s), scissors.
Step one GO! with your chosen fabric run a stitch along any frayed edges just sew (so, Ha!) the fabric doesn't unravel. Hem the material by folding it over onto its self and stitching.
Next place the material pattern side up and fold down the top section onto itself (You can make this section as big or small as you like, I'll be roughly folding mine into thirds) This piece of material will be your flap, measure each end so its the same length and will sew straight.
Following this, take the bottom of the material and fold completely over itself until what was the bottom touches the top part of the flappy bit.
This is when you get your folded tube envelope pancake. All you need to do now is sew straight down either end of the tube. All through in some places three layers of material.
*sew sew sew* AND *Turn inside out*
So choose your button and hand sew to the material underneath the fold in your chosen location. Then snip a smaller than your button size cut above the button on the outer fold, quickly stitch round the cut to stop fraying, and your button is donski!
Stuff your cushion in and there we go! new cushion cover for less than a quid and no more than 1/2 hour of your day. ENJOY!