here's a little tutorial
the idea is mainly to retrieve an old zoom which was useless because of poor quality.
what you need:
a bottle cap (I used one similar to that of the photo)
a lens on a Zoom recovered or purchased at electronics stores
you have to disassemble the the "objective" (the poor one are easy), then retrieve the wanted lens
to know if the lens will do the trick, make the test as in the photo
the rest is history, the photos speak for themselves