1) Place the ball in the hand-That is you have to select 3 fingers it is according to your wish some use thumb, fore finger and middle finger to spin others use fore fingers and middle fingers to spin. Choose it according to your wish.
2) Spin the ball-Spin the ball in your hand from left to right like in clockwise direction. You have to practice the spin in your hand. Better spin means it can create good angle to make the batsman confused to play.
3) Practice-Now you are able to spin an off spin but it never means that you can bowl it clearly you have to practice to get a good line and length. A good line and length means you have to bowl just a wide from the off stump so that the ball can able to take the middle stump at the time spin. And also make sure that at the time of deliver you can release the ball with spin without affecting your hand to make mistake.
4)Take best line and length-That is you have to concentrate on getting a good length area to bowl and also concentrate on getting a line which is just wide from the off stump. Be careful that don't bowl an off spin bowl in the middle line because it will make wide.
Tips:It is necessary to practice the off spin bowl like in the way of trick the batsman to play a shot so that the wicket may get. And also when you deliver a bowl make some speed variation in the different balls so that the batsman may get mistime to play a shot.
Practise the bowling with the help of tutorial you can also refer in the google for more tips about off spin and its bowling technique make sure that your ball is getting a good spin to break the direction. And also it is necessary to keep the batsman in tight line and length. All the best.