Carefully.
You have to make a sector in the level and tag it with a number. Then you have to make another sector outside the level. Select one line on this, then give that line the same number as the sector. Next choose the linedef action. There's a submenu of FOF's, so this shouldn't be too hard. Next, set the size of the sector woth the tagged line to what you want the size of the floating platform to be. The texture on the tagged line decides what the texture on the platform is, and the flats of the sector decide the flats of the platform.