Place the flange of the rubber bell end of plunger into the toilet until you feel it seat on the bottom of the toilet bowl.  Pushdown slowly at first to force the air out of the bell.  After the first slow downward push you can start pushing up and down more vigorously, keeping enough water in the bowl to cover the plunger.  Remove the plunger to see if the clog is cleared and water flows out. It may be necessary to repeat the procedure several times to clear the clog.