The first thing to consider when installing air conditioning ducts is layout. You will want your ducts to reach every room, and they must be able to blow cool air into these rooms from a reasonable location in the room (usually from the floor or ceiling). Each duct will end in a vent that is installed in the ceiling floor or wall, so the duct should be positioned somewhere that makes sense for cooling efficiency. Layout also needs to make sense in terms of where the ducts are routed through the structure, i.e. you don't want ducts wrapping all the way around a room just for the sake of installation in an efficient place, because you could minimize efficiency as the air travels through the duct.
Prepping the Ducts
To install the ducts you first need a clear path from the air conditioning unit, through the ceiling or floor, then to the vents in each room. If your building was not built with room for ducts, you will need to make room for them. Most of the ductwork should be installed in between the joists so that you do not have to cut through them in order to install the ducts. There are various codes and regulations for installing heating and air conditioning ducts, and each state has different requirements for residential and commercial buildings. Consult such pertinent codes before you begin.
The purpose of ducts is to circulate air from the air conditioner into the room itself. This is accomplished simply by moving the air through a solid metal structure, the duct. Most ducts are made of aluminum, which is very light and thin, and most are installed via braces and screws, attaching to the joists under the floor or in the ceiling. In some larger buildings that require more airflow but have less room for ducts in the ceiling or floor, the ductwork is exposed and mounted to the ceiling itself.
Hiring a Contractor
When installing the ducts you will have to rip up floors or take down ceilings, so hiring a contractor to ensure the job is done right is a good idea, especially if your home was not built for central air conditioning. Furthermore, a good contractor will also be able to establish the best routes for the ductwork and choose the proper-sized ducts so that you have a more efficient cooling system. Don't expect to save a whole lot of money by doing the job yourself. Any money you save will be offset by backbreaking labor, and you could actually cause damage to your joists and home (not to mention your person) if you cut too far into the wrong joists.