I just converted an 8x10 metal shed. Bought it at home depot a few years agofor about $200.
I framed a wire door inside the sliding double doors and leave them open, thus having a 5x6 window. I cut some ventilation grills into the walls and cut a 3x4 window into one wall. I still have to paint the roof white but even with the brown roof it is suprisingly nice at 80 degrees. I'll also be cutting a turbine into the roof. Cutting the metal is easy with tin snips but then you have to frame the cut openings. I used 1x4 on both sides with hardware cloth sandwiched between. Really the biggest problem is not having anything to nail to on the walls but I worked everything out by using the metal frames and some screwed on 2x4 and 1x4 supports.