Step 1: Print or buy your wallpaper!
Step 2: Measure the height and width of the wall(s)
Step 3: Cut the wallpaper to size
Step 4: Fold down one edge of your wallpaper, insert it in the house with the folded edge in a corner, smooth out and crease opposite corner. (The reason for this is that your house will look much better if you "wrap" your wallpaper around the corner instead of having the pieces meet in the corner. When the pieces meet in the corner, it forms a slight "gap" that doesn't look natural.)
Step 5: Doodle some glue around the wall, paying special attention to corners and openings as these are the most important parts to have securely glued. I have found with the medium weight paper I use for wallpaper that you do need a thin layer of glue covering the entire wall. Otherwise bubbles in the paper appear. Some people prefer cardstock wallpaper and only a little glue on the edges. It's your choice. Keep in mind how you will hang pictures... since I usually use Velcro so that the pictures are moveable, the wallpaper needs to be securely fastened to the wall. This is messy, so don't be afraid to get messy! Just smear two or three or four fingers around in the glue!