REMODELING SHED TO INCLUDE GREENHOUSE

I have been using an old porch that I upgraded to "garden shed" status. In addition to this and for hardening off purposes in the spring, I have a very small "hot" house in poor condition. I would like to add a south-facing greenhouse to my existing garden shed. Can anyone give me some practice advice/ideas for how this can best be done? What about the advisability of adding a door into the attached greenhouse? How much ventilation will I need? It's Zone 4 here in SW Montana. Thanks in advance for your suggestions.