Open up the window fully. Inspect your window screen to make sure there are no holes or damage. The window will be open, and you don't want uninvited insects. Repair or replace the screen as necessary.
Set the fan on top of the window sill. Most window fans sit right on the lower track of the window. If you are placing a standard box fan in the window, place a block of wood on the sill ledge no higher than the window track. This will prevent the front of the fan from tilting toward the room.
Hold the fan upright and close the window over the fan. The window sash holds the fan in place. Some window fans have side extensions like those for window air conditioners. Extend the side extenders as necessary for a secure fit in the window.
Plug the fan power cord into the outlet closest to the window. You may need to move the fan to another window to find a closer outlet.