I lightly scraped the inside of the rib joints with a curved scraper in order to prepare the surface for gluing on the paper reinforcement strips. For the paper I use 90 lbs hot press water colour paper cut into strips 7 mm wide. Their application is sticky and tedious. I soak each strip for a few seconds in the hot water bath that I use to heat the glue. My "glue pot" is visible in the upper right of the photo. The glue is a thin hide glue made from pellets or "pearls" that I buy from Lee Valley Tools. I apply glue generously to both the water-logged paper and the joint. After pressing a few strips into place I clean up the area with a hot damp cloth. When finished, I insert a temporary "false belly" inside the top edge of the bowl. This keeps everything stable while the glue dries.
Learning from Mistakes. The second lute that I built was made with 21 (I think) maple ribs without spacers. I found the construction terribly difficult. One day I found that it was easier than before so I worked late. When I finished the bowl I was elated. But when I glued in the paper strips and returned after going home for the day I found that the outside of the rib joints had opened up. Not separated but puckered up -- open on the outside but firmly glued on the inside. My beautiful joints were wreaked. Since then, I cover the outside of the bowl completely with plastic wrapping tape before I remove it from the mould and I leave it there until the glue from the papering is dry.
It has gotten late and I want to dig up some old photos to make my point. So I will continue in a day or two.