You might wish to get a laser level. We've hung borders in both of our kids' rooms and it was wonderful. We also used it for two-tone painting and for hanging pictures. It isn't terribly expensive and although you won't use it a whole lot it will really save you a ton of frustration.
We used traditional borders. Started in the bath tub by submerging/soaking them, then brought them into the bedrooms to hang. I also kept a basic kitchen sponge handy during the hanging to push out the bubbles. With the laser level, all you have to do is slide the top of the border to match the red line and it'll be even (provided you sit the level on a piece of furniture that is level). We went with the traditional border so that we had a little 'wiggle' room when placing it on the wall; if it wasn't in the right place, it doesn't adhere immediately and we could just slide it into position. I'm not sure if the peel and stick ones can be moved/removed as easily (never worked with them so I don't know).