When I did the wallpapering in my Pierce bedroom I made templates out of paper (including the bump out areas, which I then transferred onto cardstock. I applied the wallpaper (scrapbook paper in my case) to the cardstock with spray adhesive, and then I glued the cardstock with wallpaper onto the walls with regular wallpaper paste. It took a lot of trial and error to get those paper templates to fit exactly.
The trim was a whole other issue. I kept forgetting which side I had decided was up and cut SO MANY pieces of trim wrong. After lots of swearing (and maybe some drinking) I finally got all the pieces cut correctly and glued together. I then touched up the corners with latex wood filler and painted the whole thing in one piece before gluing in. It's definitely finicky work, but now that I look at it I think it was worth all the headaches.