Flow synthesis in microfluidic devices has been rapidly adapted in the pharmaceutical industry and in many research laboratories. Yet, the cost of commercial flow reactors is a major factor limiting the dissemination of this technology in the undergraduate curriculum. Here, we present a laboratory activity where students design and fabricate homemade microfluidic flow reactors using thermoplastic molds and soft lithographic methods, and conduct inorganic syntheses of magnetite particles and gold-citrate nanoparticles, as well as a two-step organic synthesis of azo dyes. The success of these reactions can be quickly assessed by colors or physical characteristics, which makes optimizations of reactor designs and reaction conditions possible on-the-spot. The activity also successfully integrates some important concepts in chemical engineering, organic and inorganic synthesis, nanomaterial chemistry, and analytical chemistry.