This comes pretty early on in your first game which is partially a tutorial in the beginning stages. In the single player campaign, you will be introduced to your engineering staff after completing your first research on body armor. The engineer will ask you where you want to allocate your engineers for production. Simply pick the body armor that you just finished researching and make at least one (or as many as you can afford for your soldiers). This should be completed without any waiting time after making the selection and then the achievement will pop.