I know this may be more than you asked for but I'm going into full detail just in case anyone else may be wondering too:
To use a 1 1/8" stem on a 1" steerer you will need a 1" to 1 1/8" shim and a 1 1/8" top cap. If you plan on using a spacer on top of the stem, that needs to be 1 1/8" as well. Any spacers below the stem must be 1".