If you have pvc sticking out of wakk you cut it off about 1″ in front of wall, glue on a 1-1/2″ trap ad. install garbage disp. and connect up trap, you might need some slip ex. pieces , if you have 2 bowls the gd. goes on the bowl side where drain is and the other sink drains into the gd. waste with a twin waste.
Look on this site and ask how to install gd. http://www.google.com