By locking onto belt tensioner pulley's nut and turning clockwise, you'll be forcing the tensioner to move in a clockwise direction with the wrench. From there you just hold the tensioner back in that position and slip off the belt from the tensioner pulley. Now you can slowly release the force you have on the tensioner.
After the belt is off the tensioner, there's enough slack to remove the belt. To put the beltback on, just reverse the process.