How to install a window to replace AC unit?

Jacob, great demolishing job:-)! anyhow, at this time you will have to repair the brick work in the opening. Use what looks like a top beam in your pictures as the lentil. Get some pressure treated lumber and fasten it with anchor bolts around the window opening. That will give you something to mount the new window to. I would check sites like this for installation instruction. Of course, not knowing where you live, you also want to make sure that you have proper drainage away from the window etc. Follow your local building code. It is a relatively small opening, so keep the framing reasonable. The attached images shows how it should be done:-)Hope this helps, good luck.

Do a Google search for transom+window and images for some ideas.