When your US Flag is displayed on a short staff or on an indoor flag set where it cannot be lowered to half-staff the proper way to indicate mourning is by tying a black bow above a full-staffed US flag. This black mourning bow measures 16" and is made from a sturdy black nylon material.
I've made mourning bows before but it' hard to find the right kind of ribbon which is expensive for outdoor use quality. Worth the price. I mount them on my windsock American flags when flags are to be at half staff.