How can I shoot a low-key portrait outside during the daytime?

For low-key portrait photos, one requirement is a dark background which is not there during the day outside. I tried to shoot one, but it seems I do not know how to do it. I am using a Canon 7D, 430EXII, and YN600-EX-RT. All the shots that I took had a bright background and my settings were

F/33, 1/250, ISO100 lens: 50mm f/1.8

So in summary how can I shoot low-key during the daytime?