On my RV-12, I disconnect the outflow fuel line at the gascolator and install a temporary fitting with a short aluminum tube that feeds into a fuel storage container placed below the gascolator. Intermittently use the electric fuel pump to empty the tank - take your time. Be patient with the emptying process as the electric fuel pump can empty the tank's internal reservoir faster than the fuel can refill the reservoir. You can tell when the electric fuel pump is just pumping air by the change in its sound. Repeat the process until no more fuel flows out. As Randy noted above, there will be some residual fuel in the tank when you unfasten the two tank bottom fittings - be prepared!
Be sure to take all appropriate safety precautions (e.g., bonding/grounding, fire extinguisher nearby, out of the hangar, etc.). Of course, it also helps to fly the plane before the draining to reduce the amount of fuel you have to drain.
There is a lot of discussion regarding this in past posts - try a SEARCH.