To place an object the first and most important thing is to prepare its properties at the appearing settings bar at the bottom of the screen. There you can set its dimensions as well as you can choose its object type.
When you have set the required object type and its properties, then tap its location on the floor plan. As the object appears you can tap and hold to move it, or rotate it using two fingers (keeping their distance unchanged) as you would move or rotate a peace of paper above the floor plan. When you tap the screen at a different location, where there is no current object placed, then a new object of the current type will be placed down.