Print out the diagram from the software's File menu. Print it out on a glossy paper, such as magazine paper. You should ensure the circuit is mirrored before doing that (most PCB layout programs have this as an option when printing). Once printed, make sure you don’t touch the ink part on the paper as it can get on your hands.
Align the circuit diagram on the paper with the circuit board (the diagram should be facing the copper part of the circuit board). Start up your iron. Set the iron on the cotton setting and wait until it heats up.
Dry the board. Get the big chunks of water off by softly wiping with a napkin or just letting them fall out. It shouldn’t take more than 30 seconds and it should not be vigorous or else the ink on to the circuit might come off.