First of all all need a millitary disguse. Next head to a base in the disguse. Once at the base look for the commander. He will be wearing a cap. Go behind him and hold rb and press b at the same time. It only works when no one is looking. After that head to the market and press y to enter the base. Reap this until the achievement pops. Go to other bases or try again at the same one after a short while. I hope this helps.