This is a very easy achievement. At the entrance of the vault, there is a box of supplies with 10 vault suits and 10 pip boys. Turning on the beacon will attract people to the vault. Once you have some dwellers, simply take the suits and the pip boys out and equip your dwellers.
The easiest dweller to do this with is obviously Clem since giving him the vault suit is a requirement. Put both the vault suit and the pip boy in his inventory. The pip boy will be automatically equipped, but you will have to make sure he equips the suit. Once he has both items in his inventory and is actually wearing the vault suit, you'll get this achievement.