We really like this idea. (We've written about how we liked it when Kristian did the same thing to his hallway.) And we're tempted to do it inside a closet, or kitchen cabinet, or medicine cabinet. So we asked Alicia for her tips.
I had a bunch of pieces of vintage wallpaper (from my friend Pam Garrison -- sometimes she has it in her Etsy shop) and I cut them all out in squares and rectangles in various sizes with a rotary cutter on a self-healing board. Then I just gathered them up randomly, and bought some wallpaper paste at the hardware store and used a sponge brush to slap some paint on the paper, one piece at a time. I smoothed them all out with a pastry cutter, which was all I had (I don't recommend this -- the paper is very fragile and I kept ripping it, but I know they make smoothing tools for this -- I am just a very impatient person and wanted to get it done). You just kind of keep going along from top to bottom, randomly, until it's all covered. Some of them overlap, which is fine.
You could obviously do this with any wallpaper, though, if you found a book of samples you liked, etc.