The easiest way is to install 3.5" batt insulation
One of the best ways to reduce sound transmission between spaces is to add a mineral fiber batt insulation in the stud cavity (Roxul or other similar product). This will reduce the transmission of average frequency sounds. To further reduce sound transmission, the addition of a steel furring channel perpendicular to the face of studs prior to installing the gypsum wall board is recommended. Alternatively (or in addition to furring channels) there are sound dampening (higher density) gypsum wall board that can be installed, or the gypsum wall board can be installed 2 layers thick.
Install Quietrock panels instead of using standard for effective soundproofing.