Tilt head to one side and use exhaust gas from 2nd stage into open ended DSMB.
This is how an instructor encouraged me to do it, as opposed to oral inflation through the valve, the reasoning being that things never get worse by keeping your reg in your mouth. However, I was often unable to keep my head tilted in an orientation in which the bulk of the exhaust bubbles entered the SMB as intended--in other words, it was sloppy. Eventually I gave up and went to oral inflation through the valve. With my lips around the valve, all the air goes where I want it to, and it's as easy as blowing up a balloon to control how much air goes in.