Investigate the energy efficiency of an evaporation cooler. Compared to a typical refrigeration form of air conditioning, an evaporation unit uses 1/10 to 1/3 the amount of energy. Evaporative coolers are definitely more efficient.
Evaluate your cooling needs. Decide whether or not you need something as simple as a single room evaporative unit. This is basically the equivalent of a single room air conditioning unit except it is based on evaporative technology and has the same energy coefficient advantages. If a single room unit will suffice, consider options and purchase one.
Decide if you home is configured for the use of a larger evaporative unit if a single room unit will not suffice. Basically, this has to do with whether ducting can be easily set up that will establish cool air flow and ventilation.
Explore in greater detail the design and construction of evaporative coolers. Consider whether or not you want a one or two phase unit--two phase units are less energy efficient and more expensive but do not introduce humidity into your home.
Consider the extra consumption of water that is necessary for the use of a unit. This factor is often forgotten relative to both cost and use of resources.
Decide whether to use a ducted unit or a central unit. Compute the number of square feet to be cooled. Decide on the size unit that will be needed. Decide whether you will install the unit or have it done.
Learn about maintenance of an evaporation unit--how it's cleaned, how it's winterized, and how its electrical systems are maintained.