I assume you want it done right. You want it to operate efficiently. You want the system to last.
The first, is to see what can be done to make the home more efficient, such as adding insulation, correcting window propblems and taking care of any infiltration propblems.
The second, is to do a heat load on the home to verify the size system you need.
The third, is to check all the ducting, that it is the proper size and if it needs to be sealed, upgraded or replaced to meet the requirements of the A/C systems cfm and static pressures.
The fourth, and most important, is to find a good HVAC company to advise you and to install the system.
It takes many years and experience to know the proper way to install a system correctly and this is not a weekend DIY job unless you don't care how it is done or if it will work properly.