re-align your callipers to your discs so that they run parallel and dont rub:

you dont need to take the wheels off, just unscrew the brakes and let them hang on the cables.

now cut two small pieces of card and place them in the gap where the disc goes. before putting them in fold out the edges a bit, so you can put it over the disc easier. now start to screw the brake back onto the holder thing (i dont know the technical word for that part )

it should be quite a tight fit... if not, take it off and use thicker card or fold it over if thats not too thick.

take the card out once you're done and there should be an even gap either side of the disc