In the bat cave, head up to the evidence section and choose the Riddler case file. There are three separate subentries, each of which will create holographic images of Riddler gear in pieces. For each one, you have to rearrange the pieces until they click together. Unfortunately, they tend to drift so it's hard to put one exactly where you want it while fiddling with others, but once attached together they don't come apart again. All-in-all not hard, but sometimes not obvious how the parts go (particularly the cube). Once all three are assembled the achievement will unlock.