The idea is to hit the person just below the knees with your shoulder because if he doesn't have his legs he won't be able to move. Also go in to the tackle hard because if you don't it is going to hurt you more. Here is another tip when playing in a game introduce yourself to your opposite number then the scrumhalf then fly half (when i say introduce smash him) especially useful for back row players. so you go down on youre knees hip shoulder