There are two things to consider when you do a throw-in. First of all, of course, you have to be out of the field, off the field, behind the sideline before you toss it in.
But before we get to the technique I have the urge to just emphasize that when you have a throw-in, the idea is to not just toss the ball anywhere on the field. Actually you have to observe what's going on the field, the behavior on the field of your strikers, of your midfielders is wherever you're on the field who gets open, who gives you a sign to pass the ball or to toss the ball to you, throw the ball at you. Or who's moving into the open space. So the idea is to, of course, get an advantage and not see it as a disadvantage because you're using your hands, which is the part of your body you have most coordination with.
So having the throw-in, aim it at the player you want to pass the ball to into the foot, to the chest, not anywhere, ideally to the strong foot so he or she has an easy touch back to you or as an easier way to control it for himself, for herself.
Of course we want to have both feet as we said earlier behind the line. But it's more about the upper body to get momentum behind the ball. We rise the ball up, use both hands, get the ball between the hands and almost build a triangle behind it so we have enough power and control and the ball's not slipping out of our hands when we actually throw it. We lean back with the shoulders going behind the head, lean all the way back and then throw it out of your hands.
Remember to focus on your teammate more than anything else. You want to have an advantage and not a disadvantage out of the throwing.