How can I fit an air conditioner in a basement window?

I have a basement apartment with small windows. I already have an air conditioner that fits in the space, but I don't know how to close it in the window. Does anyone know how to make an AC stay in a basement window?

The window is not a sliding window. It lifts in and has a pole to hold it open.

You may have to fabricate some pieces out of wood to make the sill plate similar to an above-ground window. the unit should be balanced so that it will sit in the window and allow pieces of wood, cardboard, plexiglass or rigid foam to fill in the gaps.

You would take the window out.

You will have to do a bit of work to make an area for the AC to rest on. The AC will rest on the window frame, You can insert a couple of boards under the AC where it will rest on the outer sill. Make tilt just a little to the outside.

You can fill in the open areas around the edges with Styrofoam or plywood or what ever you think will work.

One thing that is important is to make the AC is secure so it can't be pushed in or out. That may take a bit of thinking.

buy a measuring tape and measure for size and then go to the store

You need to make a shelf for it to rest on. This shelf should have a very slight slope to it towards the outside to assist with drainage.
Make sure you seal the window with a board if the AC is smaller than the opening. then tape it in place.
Use duct tape (or masking tape) to create a seal along the sliding portion of the window, so you can still open it for fresh air.

some wood for the space, and just close the window down to hold it.

get some 1 1/2 Styrofoam and you can cut it with a hacksaw blade and squeeze it in place for a air tight and insulated fit. Good Luck