Getting black glue lines on projects? Metal containers like coffee cans can cause this problem, which occurs more often with white glue than with yellow glue. Glues with a pH lower than 7 can absorb iron from the metal, and the dissolved iron can react with certain colored woods to leave black glue lines. Use plastic containers instead.
Wallpapering can involve a steep learning curve and most professionals advise starting in large rooms. Save bathrooms for last, or at least until you’ve got your confidence.