How do you install an air conditioner in a casement window?

If the window is VERY wide, use a supporting 2x4 cut to fit inside the vertical channels and close the window to the top of that. Be sure to move the unit toward one side of a very wide window and use a panel to block the openingn on the other side.

Also, support the bottom of the unit at the outermost sill with another price of lumber cut to fit.. Window acs are not designed to be merely hung by the top and bottom front edge.