The first step, was to start building the front wall and (round) door frame.
I dug down to pour the concrete for the foundation.
Once it was poured and smoothed out, I put in a couple of brackets to hold posts.
When it was all dry, I placed a large piece of OSB, which I had cut previously, flush against the pipe. It is just resting on the foundation and will be held flush against the pipe by the wall structure to be built in front of it. Having that basic shape to start with, suggestive of a hobbit hole, really motivated me, even though this piece will be nearly completely covered up by the time I am through. One crazy problem I have had to deal with is that the pipe is not completely level or square. The top lip on this end is about 3 inches further out than the bottom. There was no way to fix that short of bringing back the crane and lifting it up an inch or so on this end, and possibly cutting part of the lip to make it square. I decided I would just deal with the crookedness. Besides, it gives it more character. (When I get to the back wall, I will have the same problem in reverse.)
Next I installed the first two posts, and built the wall up a bit with cinderblocks. This will all eventually be hidden behind other wood, stucco, etc. Because of the 3 inch variance mentioned above, getting those posts in with the cinderblocks around them was a real puzzle. It took me most of a day just to figure that out, but as you can see, I finally got it together. The posts are totally level. The top is tight up against the OSB, but the bottom is about 3 inches out. That's why the posts could fit inside the cinderblocks.
To complete the basic structure for this wall/door opening, I put two more posts in front, cemented into the front holes of the cinderblocks, and secured two cross beams across the top. I also attached boards vertically to the posts on the two sides. This structure is rock solid, and totally flush with the pipe. I sealed the joint between the pipe and the OSB with Great Stuff. (Additional water/weather-proofing would come later.) I also put in a threshold, which is bolted into cement in the cinderblock cavities.