How do you install an a/c window unit on a metal window frame with a lip sticking up from the ledge?

If you have a difficult situation it's nearly impossible to tell you how to deal with it unless you can provide detailed information, drawings or pictures. But I can tell you how to approach it. The weight of the unit needs to be carried on the bottom of the window opening, a sill or something like that. Then the air conditioner usually hangs out of the window, which means there has to be a board across the top of the unit that bears on the inside of the window. I made two boxes like that, and it takes care of my little house. I can put a box in a window, it has the air conditioner shell screwed into it, and then I slide the mechanicals into it. Weight alone holds it in place. I cut a piece of insulation board to pop in each side.