You can't. Simple as that. A good side step can't be defended. The attacker has the advantage because the defender has to react. Watch the '73 Barbarians/All Blacks game. Everyone always raves about the sidesteps that Phil Bennett did to start off the famous try. But I always like seeing the running of David Duckham. Watch the run he makes after doing a scissors with JPR Williams. A big fast winger that could sidestep at speed. Unstoppable. In an International against France Duckham once stepped the French full back so comprehensively that the guy put his hands to his head and patted Duckham on the back as he jogged back after scoring.
Watch their feet, that's all to it. You'll see where they're gonna go and BOOM! You've smashed them.