You don't!  At least, according to common lore.  Most people sug-
     gest you show the person in question your affection.  But, if you
     want to ignore this advice, you can try your luck with "ai  shite
     imasu"  or "daisuki desu".  Since you (usually) say this to some-
     one you know (quite) well, it is probably more  natural  to  drop
     the  politeness  level a bit and use "ai shiteru" or "daisuki da"
     instead.  To add some emphasis, you could add  a  sentence  final
     "yo".