Many people think this is the way to "stand properly" and this is misguided. There are some yoga poses that call for this action in the right context, but it isn't a neutral position or one to strive for in daily life.
Let your shoulders lift a little and float, rather than pushing them back or down. Allow the shoulder bones to reside in the center of the shoulder sockets.
When you push the shoulders down and back, the muscles and fascia of the neck and upper shoulders can get pulled taut. Floating, centered shoulders allow the soft tissue to be slack and relaxed.
1) Stand with your legs under you for support. Soften and relax. Breathe into the shoulder area. Let go of any "doing" or "fixing" in the shoulders.
2) Lift your shoulders up towards the ears like a shrug. While shrugging, wiggle your shoulders in many directions. Now try moving the shoulder bones way forward, back, down, and in many directions. Feel all the possibilities.
3) Now relax the shoulders without pushing them down -- just let them surrender to gravity. Create a subtle buoyancy in the shoulders. Explore a neutral place, where the shoulders aren't pushing down, forward, back, or up -- Where do your shoulders feel centered?