I had some leftover styrene 'metal' pieces from a Rix Products Quonset hut kit to use as the form. They're curved, so I carefully flattened them, glued a few together, and then glued the assembly to a piece of wood with super-glue.
Tape a piece of paper squarely on the form. I don't know what the weight of the paper was that I used, but it felt similar to photocopier paper. Don't use anything too heavy, or the embossing won't be deep enough to be seen. As well, if the paper's too thin it will tear during the embossing process. The simulated metal sheets were to be a scale 4' x 8', so size the sheet so that you'll get whole 4x8 panels when you cut it up.
Strangely enough, trying to find a suitable empty pen around the house took a little doing. Most of the ball-point pens I could find had a tip that was too large to fit into the grooves of the form. Eventually I settled on a purloined Hilton Hotels pen still filled with black ink. No problem, the panels ended-up having black lines from the embossing process on their back side, but once they were glued in place and painted, they weren't visible.
It took a bit of practice to make some good looking panels. Use a steel scale to help guide the pen in the grooves to keep things aligned. Don't press too hard or you'll tear the paper. Go slow. The later panels I did, which are the ones used on the dust box at the back of the complex, turned out the best. They had consistently straight, well-defined grooves. The ones used on the addition aren't too bad, but they have a little less definition. One trick I learned was to make a few panels, then put off making more for a day or two. Coming back to the task in a different frame-of-mind seemed to help reinforce the skill for some reason. I'm thinking of building Mr. Moore's Jones' Chemical Company project from the March '74 issue of Model Railroader later in the year, so hopefully all this practice won't go to waste.
After cutting the embossed sheets into scale 4x8 panels, they were attached to the addition by coating its walls with a thin layer of white glue. Apply them in rows and make sure the alignment of successive rows is staggered so that the vertical panel divisions don't all lineup. Try not to press the panels too hard while you're gluing them in place so not to soften the definition of the embossed pattern.
[Paneled dust bin]
Once the glue had thoroughly dried, I trimmed off the overhanging pieces with a sharp knife. The addition is brush painted with Tamiya's XF-16 flat aluminum. Once dry, I gave the addition a wash of Model Master Acryl flat black thinned out with Model Master Acryl Thinner. This both dulls the shine of the aluminum paint and partially fills surface indentations with black colour to help them visually standout a little more. The barn was brush painted with two coats of Tamyia's XF-9 hull red. I try and use acrylic paints wherever I can in my work since they'll cleanup with water, and they don't seem to out gas a great deal which makes them acceptable for use in the house.
Mr. Moore recommends using balsa to make the foundation and that's what I did. But when it was done, and I read the paragraph again, I think he meant it to be built up from complete sheets instead of strips framing the perimeter. That would have made it squarer and more solid, as well as providing a floor for interior details. The finished foundation is painted with Tamyia XF-20 Medium Grey.