If you're the DM, you can just tell the players when they level up, whenever you feel appropriate. If the players don't like it, too bad for them. Unless they REALLY don't like it, and aren't having fun, in which case you probably shouldn't do it. Still, if you just want slower leveling, then just cut all XP awarded in half, or by whatever percent you want. As long as your players are having fun, you can do pretty much anything you want as DM.