I jumped ahead to Step Q.h which is applying the foundation trim, and then applied a paper-clay masonry effect around the foundation. I used my Dremel tool to bevel the joins of the wood strips around the bay, so they looked more like mitered joins. I found one of the pieces on the right wall was about 1/4 inch short, so I filled the gap with a bit of scrap wood from another sheet. A bit of filler smeared along the top edge, and in the joins, makes these trim strips look pretty good.
This is the first time I've used paperclay - it is actually Das modelling clay because that is all they stocked at Hobbycraft - but found it quite easy to use.
I used a plastic trowel to mark out horizontal courses 3/4 inch apart, and vertical 'stones' one inch wide. Some of the stones I split into two horizontal stones. I pounced all over with a stencilling brush to give texture. Then I applied a thin film of wood glue to the foundation, and pressed on the clay, using the stencilling brush to prod the clay onto the wood. The cellar windows were cut out of the clay after it was applied, while it was still wet. I also pressed the cellar sill and window frame into the clay to create depressions where I can glue them into later.