I'm messaging here as this forum provided the best information when I was thinking about doing this. So in case it helps anyone else, here is my recent experience of wallpapering with OS maps.
Before I started I did a test using an old Explorer map and ordinary wallpaper paste. I decided not to paper the entire wall from skirting to ceiling and from corner to corner, as I thought this would be too complicated and expensive. Six Explorer custom-made flat sheets cover most of the wall in my study. It looks fine, with the surrounding border painted in the same emulsion as the rest of the room.
One of the hardest things was ordering the maps to cover the area I wanted, centred on my home town, and minimising wastage. Unfortunately it's not possible to order maps by grid square. As I was papering with 3 sheets above 3 sheets, I ordered the upper centre sheet first. Once I had this it was easier to set the edges of the adjoining maps to fit closely.
After trimming, I hung the top centre map first, positioning it to a top horizontal line pencilled lightly on the wall. I used ordinary wallpaper paste, allowing the paper to soak for 2 minutes before hanging. It does stretch quite a lot: about 2 cm across the width of an 80x80cm sheet. I am not an experienced wallpaperer, and there were quite a lot of wrinkles. As the paper dried it contracts and I was able to smooth out most of the wrinkles and air bubbles, with the help of a soft cloth and a needle. The detail on the maps means that the few remaining wrinkles don't really show. I butt-joined the maps and used a roller over all the edges.
I've varnished the maps with Rustins water based floor varnish. This was just something I had left over from another DIY job, and the test map had showed that it worked really well. I could even used a highlighter pen on the maps, and then wipe it off (though I probably won't try this on the finished wall!)
The whole project including ordering, trimming, hanging and varnishing has taken about 6 hours and cost just over £100. The area covered is a 240 cm by 160 cm.