I took these photos of a wallpaper installation my poor friend did of an old ferm pattern about 5 years ago. I've finally moved past the heartbreak and feel strong enough emotionally to post them. So here we go...Hopefully you can learn from her mistakes and avoid this wallpapering tragedy.
1. I thought this went without saying, but match the pattern repeat.
2. Unless you're some kind of pro with a craft knife, use a straight edge to cut the trim.
3. Don't cut the wallpaper horizontally.
4. Sponge off the wallpaper after you hang it to remove any excess paste.
5. Use a sharp blade to cut the trim. If not, you end up not getting a clean cut. It also helps with this to use a straight edge. See 2. above.
6. When cutting around outlets, light, curtain rods, etc., don't leave any space. Ideally you should take them down, but if that's not possible trim right up to the obstacle
I promise, wallpapering it totally doable. Just follow the instructions and try to have a good time with it. And now, at least you can rest assured that someone out there did a worse job than you ;)