Seoi-nage is accomplished by making your opponent off balance by pulling in a straight forward motion, or a right or a left forward motion, then pick up your opponent's body on your back and throw over the shoulder.
Generally Morote-Seoi-nage (two hands Seoi-nage or Eri-seoi) is called Seoi-nage. Technically tori grips with his two hands.
Tori & Uke hold each other in a natural right-handed posture. Tori pulls the Uke straight forward while stepping back slightly to pull the Uke off balance. The Tori then places his right foot in front of Uke's right foot, starts to lift the Uke with both hands.
The Tori bends both knees, spins on his right foot while dropping his hip and placing his left foot in front of Uke's left foot. The Tori and Uke are now back to front. The Tori bends his right elbow and places it in the Uke's right arm pit, then pulls the Uke's right arm down and locks it against Tori's body. While keeping close contact to Uke's chest, lifts Uke up (picture 1).
Tori lifts Uke with both hands and bends his body forward, while straightening both knees throws Uke over his right shoulder. With Tori's right arm acting as a shaft, Uke's body will spin and fall (picture 2).
Techniques of practical applications
*This is a practical application of Seoi-nage technique and not a Tai-otoshi technique.