Here's the rough opening with the door removed.
So that I could have something reasonable to attach to in the doorway I added a set of 1 x 4's. It functions in this case a bit like a door jamb.
Here's the top end with the 1 x 4's.
I made this frame for the door itself a couple of days ago. The 2 x 4's are recycled from another site. I used half lap joints at the corners. Unfortunately for me I don't have the means any more to make finger joints which would have been better.
Here's the doors framework filled with the styrofoam insulation fitted inside. I think this material is about an R-6 and I left the vapor seal intact on purpose.
A few hours later here it is ready for use. I still have some tweaking to do so that it will seal even better and operate more smoothly. It needs a coat of paint too.
Later I have plans to add a window in this door near the top. That's for later on though. I'll be double glazing it so that it will the most energy efficient it can be.