In this throw you put your opponent on your back, twist and then throw them. The seoi nage is a very powerful throw often used in competition and it is among the most important throws in judo.

For a right side throw, grab with the right hand on the opponent's left lapel, and grab with the left hand on the opponent's right sleeve at the elbow. Pull both hands strongly forward and upward while entering close and low, turning your back to the opponent. Without letting go of either hand, place your right elbow under the opponent's right shoulder and roll him/her over your right shoulder as you twist to the left and straighten your legs.

1. Put your right foot forward, bend at the knee and pivot on it. Pull your left hand back and turn your body and your head to the left. Bring your left foot close to the inner side of your opponent's left foot. Your toes and their's should be facing in the same direction.
2. Keeping your grip on their left lapel, turn to the left. Your wrist will natually bend inward and your hand wrap under the cloth of the lapel. Bend your head down, and put your back tight up against the front of your opponent's body.
3. Bounce on both legs, straighten your knees, twist and bend your upper body forward, and pull straight down with both hands. Throw your opponent sraight down in front of you, letting them cross over your right shoulder.

You should see Judo Unleashed or Black Belt: Judo Skills and Techniques by Neil Ohlenkamp for a basic reference.

Here is a video of morote seoinage and here is another one.