Don't try to set an Olympic record by trying to manhandle a large air conditioner. While smaller units are okay for a solo effort, it may require two to three people to install a larger unit in the window. Try to place the window or wall air conditioner where it will be shaded from afternoon sun by a tree or awning. That way it won't have to work as hard to provide cooling. The sash supports the weight of the unit.
Heavier units can stress out vinyl windows over time, causing the sash to bow.
Make sure the unit is supported in some other way, with struts attached to the window or exterior of the house. The air conditioner should be mounted so that it is slightly tilted toward the outside so that it'll drain condensed water onto the ground and not back into the window. Be sure to block any gaps around the equipment with weather stripping or sections of foam plastic. This equipment is for those who don't like having their views blocked by a window air conditioner. They cost about the same to buy but more to install.