A huge part of the illusion here is at the top of the 'post' over which the drapery hangs. Get this right and it's impossible not to imagine the square post beneath.
Beginners can stop here with a really good looking object but I'd encourage you to go further and put in the bottom edge of the drapery and so really bump up the illusion. It's not so difficult as you might think, particularly because we can make it up a bit as we go along. Check out the next videos!