You'll need two matching embellishments, both with flat backs that are roughly the same shape and size as the flat surface of your cufflink backs.

Most cufflink findings have square or circular flat backs measuring approximately 0.4 inch (10 mm). For cufflink backs of this size, the flat back of your embellishment should be between 0.4 and 0.8 inch (10 to 20 mm).
Popular options worth considering include small Lego bricks, flat-back metal charms, coins, non-shank buttons, and polymer clay charms.
If you're feeling especially crafty, you can even layer together thin flat embellishments, like watch cog.
 If you want the embellishments to match the cufflink backs, spray paint them an appropriate metallic color. Let the paint dry before continuing.

This is only optional, of course. It isn't necessary if the embellishments already match the cufflinks. Even if they don't match, you can still leave the original color to create cufflinks with a more informal appearance.
 Apply a dot of hot glue to the flat surface of the cufflink back. Quickly center the flat back of the embellishment over the cufflink and press it into the glue.

Allow the glue to cool and dry completely before handling the cufflink.
Note that superglue could be used instead of hot glue, if desired.
If layering cogs or other thin, flat objects, apply a dot of glue to the cufflink back and press the largest piece on top. Give it a few seconds to dry, then apply another layer of glue on top before pressing the next piece into that. Repeat as needed to create the desired effect., Glue the second embellishment onto the second cufflink in the same manner.

When finished, you should have two nearly identical cufflinks.
 To put the cufflinks on, simply insert the stem of each cufflink into the buttonholes of each sleeve.
Choose your embellishments.
Paint the embellishments, if desired.
Glue one embellishment to one cufflink back.
Repeat.
Wear the cufflinks.
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