Straightening or truing a wheel is a pretty simple concept: tighten or loosen spokes to remove the side to side deflection of the rim.
To achieve the straightest wheels possible and wheels that will stay straight Trailhead Bike Co. uses professional tools such as the Park TS-2 truing stand and spoke tension meter.
However check this video out and you too can straighten wheels to get you and your buddies rolling again.
PS. The video shows the mechanic using V brakes to help true the wheel. If you have disc brakes wrap a cable tie around each seat stay opposite the rim to create a guide. Trim the end of the cable tie off so it is 3-5mm from the rim.