Breathe in gently through your nose.
Imagine the lowest part of your lungs filling with air.
When you use your diaphragm, your stomach will come out a bit, as your lungs fill with air.
Imagine the middle part of your lungs filling. As you picture them filling up, allow your rib-cage to come a little forward.
Imagine the upper part filling with air and your lungs becoming completely full. Your shoulders will rise slightly and move backwards.
Regular relaxation sessions will teach your body how to relax properly. Just listening to music or relaxing sounds will not achieve the same thing.
A few basic guidelines...