If the temperature is uncomfortably hot and/or humid both day and night, then this method isn't appropriate for you. If this is only the case during the day, a window fan can be successfully used to bring the temperature down at night.
If you live in a very polluted area, using a window fan will pull this pollution inside.
Window fan ventilation is not recommended for those without adequate window screens, as insects or animals may enter through the open windows. Gnats and other small insects often get in right through window screens, so be prepared to accept that or don't use this method.
In high crime areas, windows accessible from the outside are also likely to be used as a means of entrance by criminals.
Noises outside the home will also become louder inside with the window open, although the fan noise may mask some of this.;
, Ideally, each fan should be the largest that will fit inside the window. Avoid fans that are too large for the window, as positioning them near the window, outside the window frame, frequently results in their falling over. Your house should have an approximately even capacity blowing inward as outward (bearing in mind that smaller, less powerful fans should be counted as a portion of a fan for calculation purposes). If you have an uneven capacity that can't be balanced, it's better to have more blowing inward, as that creates a slight positive pressure inside the home, which prevents carrying dust and insects in when doors are opened and stops "bad air" from backing down chimneys.
 This will also determine which fans will blow inward and which outward. There are many considerations here:


Blow inward on the coolest sides of the house. This will typically be the sides in shade.
Work with the prevailing wind direction, pointing fans in the same direction as the wind blows, rather than fighting the wind. On days when the wind is strong enough, you may not even need the fans.
In a single-story flat, arrange fans to blow inward on one side of the house and outward on the opposite side, with the doors open in between for maximum air flow.
For multi-story houses, another option is to use the fact that heat rises to assist the air flow by blowing cool air into the lower floors and hot air out of the upper floors, including the attic (with the attic door left open if possible).
Avoid blowing inward near garbage cans or parking areas, where fumes and odors may enter the house. Air intake near trees or plants often provides pleasant smelling air, unless pollen presents a problem.
Fans blowing inward toward a refrigerator/freezer may blow cool air out quicker than usual (when open), thus increasing the load on the device, so avoid this.
Bear in mind that an inward blowing fan may disrupt loose papers, so avoid this in areas like a home office, or secure all papers first.
Avoid placing inward blowing window fans above valuable items, like an antique desk or expensive oriental rug, to prevent water damage.
Rooms with fans blowing inward will both seem cooler (due to the increased circulation) and will cool down quicker than rooms with fans blowing out.
 Close the window as tightly as possible around the fan to hold it in place and prevent local circulation. Local circulation is where the same air blown through the fan then goes back around the fan and gets sucked back in, repeating the cycle indefinitely. This just results in the motor warming the air.
 If the fan is blowing inward, the air will want to go back out around the fan due to a local pressure increase. Placing drapes or even paper at the sides of the fan will often work in this case, since they will be suctioned to the window screen. Fans blowing outward are more difficult to mask, as the air around them will want to blow inward. You could place paper or cardboard on the outside of the window, if it's accessible, or tape it down on the inside. Or, you could opt to skip the masking in this case and accept the lower efficiency instead.
 Turn them off, remove the fans and close the windows when the house is cool enough to offset the predicted daytime temperature. A general rule of thumb is that the inside daytime temperature will be the average of the outside daytime temperature and the inside nighttime temperature. So, if you want to keep the inside temp down to 70 °F (21 °C) and are expecting a high of 80 degrees outside, you need to lower the inside temp to 60 degrees at night. This will vary from home to home, but you can use this rule of thumb until you figure out the formula for your area. In this example, the fans could be turned off once the inside temperature dropped below 60.
Evaluate if your situation is appropriate for this cooling method.
Choose your fan(s).
Decide on the placement of window fans.
Place fans in windows.
Mask window gaps adjacent to the fan.
Turn on window fans whenever it is cooler outside than inside.
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