I was sorting through a drawer of mine recently and found a little plastic pot where I'd over the years thrown in all the spare buttons that you get whenever you buy a new item of clothing. And although mine are mostly boring blacks and navy's, using coloured and unusual buttons is a great way to customise your clothing, and incredibly simple! Check out this article from The Guardian for some tips on how to customise a t-shirt, though you could do the same for any garment, and abaya sleeves look particularly good:
For some more design ideas, check out the dresses and cardigans over at Ellisborough. They retail for around £30 each, but with a little time and patience you can make your own for a fraction of the price!
And if you really have no idea how to start, check out this wikiHow on how to sew a button (complete with video!).