Rim tape is an integral part of your wheel build and helps protect your inner tube from puncture caused by spokes and sharp edges on spoke holes in the rim. CST make a range of rubber rim tapes to suit a wide variety of wheel sizes.
To fit, simply align the valve hole in the tape with the hole in your rim, using the valve stem from an old tube or something similar, then stretch the rim tape over the rest of your rim. Be careful not to over stretch. Once in place, ensure there are no folds in the rim tape and that it sits nicely flat and even in the rim channel. Then, go ahead and fit your tyre and tube.