How to mount this window?

I've got a recycled flangeless window. I cant figure out where to start the fasteners to attach to the framing. I can see the holes in the window assembly from the previous install. (Where the nail is hanging in pic 1) And can hear the sheared heads of the old fasteners rattling in the assembly when I tilt the window. It seems like the panel marked in red in pic 2 might be removable? Am I on the right track or is there a different/better way to fasten this flangeless style window to my framing?