Platforming is done by purposefully creating a temporary gap or hole and clearing a "platform" of pieces above. You can find more examples of this idea in the downstacking article.
Games that start with a combination of the S-, Z-, and/or O-pieces force the player into creating a gap on the surface. Generally, players cope with this by either soft dropping or skimming a small line clear. Instead, by deliberately creating a hole, you can platform a Tetris.
You need to stack exactly four cells over the hole. Any more and there will be some upstack left over, leaving a hole after the initial Tetris.
Position the hole on whichever column you plan to create Tetrises on.