Here are some simple directions on to accomplish this task.
Using a multi-meter, preferably a digital unit, set the meter to DCmV (the 200 range, if you have to choose on your model).
Turn on the amp, turn down the master volume, and take it out of standby and place into operate mode.
Let the amp run for a few minutes. Take the red lead of your meter and place it in the red tip jack on the back of the amp. Place the black lead on a screw head on the amp. We like to use one of the four screws that hold the amp in the cabinet as those screws go to the chassis ground. Check your bias reading and adjust with a small flat blade screwdriver in the adjustment potentiometer on the back panel next to the tip jack.
For 6L6 amps, 76mV should be on target. For EL34 amps, 60mV should be on target.
Put your tools away, crack open a cold one and enjoy the sound of your amp. Remember to check bias periodlically as the tubes “break-in” and wear.