im soo happy, i have finally made some challenge without looking at the video first 
I am not sure if I done it the right way but it took me about 5 minutes. I made 2 cuts, than select the face and press E and extrude in some level, the same size level done with the second face- part of the railings. and finally using the snipping tool i connected the up part of the railings so it connect.
and of course removed doubles and checked the normals
