you'll need a camera, xacto knife, selection of bottle caps and a selection of lenses
Step 2: Select a bottle cap that will almost fit
in this case it's a prescription bottle lid from target. here I'm using the knife to trim out the inside of the bottle cap so I have a SLIGHT friction fit on the lens barrel. You want the fit tight enough so it doesn't fall off and loose enough so you don't strip gears when the barrel retracts.
Step 3: Adding the lens
After you get the fit right trim out a hole to fit your lens. When selecting a lens quality counts, pick a good multi element if you can, use plastic as a last resort, cannibalized binoculars are a great source. This is part of a microscope eye piece
Step 4: Put it all together
fit the lens in the bottle cap, slip the cap on the camera, and finally....