Usually, the casual soccer player defends by just running to the guy with the ball at his feet and trying to get his leg in between there somehere. Though part of this technique can be useful, it's of course not the best way to go. It drains the energy from you (you're running all the time), you can easily make fouls, and the opponent can even more easily dodge you and have free space to run into.
There are a few things a defender must be able to do: intercept/block, a standing tackle and a sliding tackle. Of course, positioning and marking plays a big role here, but that's described in the 'off-the-ball movement' section.
So, you've got an attacker moving somewhere near you. First, you must learn to wait. Don't rush to him, try to estimate where he will end up, and get there moving at about the same speed as him. This makes you able to either perform a tackle, or quickly run with the attacker if he changes direction or does some sort of trick. If you run towards him too fast or too slow, the attacker will be able to dodge you or move past you with a simple acceleration. Also remember, to always stay on your toes, as this makes you able to quickly change direction.
So, now you're running at the proper speed, and you're at the point where you're going to bump into the attacker. You'll now want to stand diagonally, between the player and the goal (blocking a straight line to the goal). Lower yourself a bit (a lower center of gravity gives you more balance, strength and agility - but less speed), and keep your eyes fixed on the ball, and the ball only, Whatever fancy moves the opponent makes, only the ball can truly show you where the ball is going.
Now, the attacker has the option to try and move past you left or right (cutting outside or inside). If he moves too close to you, or waits too long, you can perform the standing tackle. Otherwise you'll have to run with him and either block his shot/pass or get him off the ball with standing/sliding tackle if he loses control over the ball for a second. You're allowed to use some of your strength and body to take the opponent off the ball, but not very much, because that results in you making a foul. Other moments where sliding tackles are preferred, are when you're coming from an angle at a player with high speed, and you see the opportunity for a clean sliding*, do it! Otherwise, it can be a last resort to block a pass or stop a player in dangerous position, but be careful with that.
* This means: you're pretty sure where the ball is going to end up (and the player is not going to end up, which is the case when a player is running with the ball and can't change direction quickly.
The standing tackle is the easiest one. It can be executed if you're close enough to the ball to stick one foot out (while still standing on the other one) and kick the ball away or take it for yourself.
Standing tackles are performed with the inside of the foot, as this is the strongest part with the most surface. Basically, all you need to do is put this part of the foot to the ball, and make it go in any other direction than the attacker who first had the ball wanted it to go.
However, sometimes you'll make contact with the ball at the same time as your opponent, or he'll have enough space to move quickly and make it hard for you to get the ball. Then, you'll need to do everything to come out of this duel as the winner, because you're the defender!
Therefore, you first need to learn to close in on the attacker. Make yourself big, and make sure that whatever direction the other player is going, he'll bump into you, or you'll be able to do a standing tackle with one of your feet. Then, once you see the opportunity, lean into the tackle. Move your whole body forward and use your mass to make sure the attacker doesn't simply push you away.
And then, to give it a nice finishing touch, you can learn to determine the direction you want the ball to go after the tackle and quickly run there to get the ball for yourself. If you just tackle, most times the ball will bounce out of bounds (and you were the last one to touch it), or fly back to another player from the opposition. A good rule to follow here, is that you want the ball to diagonally move away from your goal. This moves the ball towards the sides, and not back into the feet of the attacker. This puts your team immediately into attacking mode, and you're less likely to lose the ball (with disastrous consequences) there.
A sliding tackle is a bit harder to do, and also has a few variations you'll need to master. Before we go on, a few things on performing safe sliding tackles:
Keep your feet at ground level, always. Whatever you do, any foot that reaches the knee or even higher is not only a yellow or red card for you, but also a danger for the player receiving such a tackle.
Don't hit the ball with the bottom of the foot. You may be tempted to slide to the ball, and kick it away to the side with the bottom of your foot. But, that's not smart, and also dangerous. You want to use your laces (but not your toes, watch out for that) to kick a ball away.
You don't always have to slide. Although they look wonderful when done correctly, you don't have to do them all the time once you've mastered them. Most of the times simply running your man into dangerous territory or a standing tackle will suffice. Forcing the opponent to play the ball backwards is also a win.
So, now we can go on to: how is it done?
Well, the first thing you must learn is of course the 'sliding' part. To slide, you need to have some speed, and then drop to the ground, sideways. While you do so, your lower leg (that's touching the grass), must be bend (so you're practically sliding on your knee/thigh) to get a good sliding movement. The upper leg is then stretched out and used for the actual tackle.
Another variation is where you do what I described above, but at the last moment you switch your lower leg to be stretched, and you upper leg to be bent backwards. This gives some extra power to the tackle, but has the disadvantage that you can only kick the ball far to the side this way.
That brings us to our last variation, where you keep the ball in possession. While you perform the tackle, wrap your foot around the ball, and quickly get on your feet and pull the ball towards you. This results in the cleanest possible tackle, because you keep the ball afterwards, and your tackle must be perfectly on the ball for it to succeed.
That's really all there is to it. Once you've got the basic technique, it just takes a lot of practice to get the timing, power and aiming right.
This isn't really that hard, as it just means you must estimate how the ball will be passed or shot, and then get in the way (either with your body or by sticking a leg out). However, there are a few tips and tricks:
- If the ball is shot from up close, keep your hands behind your back to prevent yourself from making hands.
- If the ball is shot from the side of the pitch, you can use your leg and body to stop or deflect it.
- However, if you're in front of goal, you want to stand still where you are once a ball is struck. This way the goalkeeper can see the ball all the way and correctly estimate where it will end up. If everyone just hopelessly tries to stop it, the ball might deflect, and that renders the goalkeeper useless.
- If you're trying to intercept a pass, don't stand precisely between the ball and the man you're marking, but a little bit further to your own goal. This forces the opponent to pass the ball behind the other player, stagnating their attack.