There isn't a great amount of literature on this subject it seems. I have an air conditioner, the manual assumes that you have a vertical sliding window, but I have one that slides horizontal, so how do I install the unit?
Most window units are designed for double hung windows. If you want to place it in a casement window (or a sliding window) you really should use a unit designed specifically for that. Here's an example:
However, anything can be hacked. If you build a proper bracket/shelf, and then enclose the top part (plexiglass, for instance) you can probably use what you have (provided it fits, of course).
Most of the ones I've seen are install like this with a timber frame made to sit into the window frame then put the air-conditioner inside of the frame and box it out with plywood, but this looks like a much neater installation where the frame is made from c channel with a perspex panel above the airconditioner.
It would probably depended on how long you intend to have the air-conditioner install in the window, as to which option you would go with.
Rolling portable units solve all such problems, The only thing that needs fit in the window, sash or slider, is the adapter for the flex duct (similar to a dryer duct) that takes the place of the AC's sitting inside the window or wall. Such units also have the advantage of portability: set them up quickly where ever needed or using whichever window is most convenient. As for casements--assuming that were all you had, the non-crank pane would need to be removed and modified to receive the duct adapter. The inside portable units also offer the advantage of easy cleaning and servicing; and an greater efficiency viz-a-viz a window/wall unit exposed to direct sun and hot outdoor temperatures; and cooling capacity which is not governed by window opening size. They are also much easier to find than vertical window units.