Take an ample piece of garden hose and enough containers to hold the fuel you are going to take out.
Put the garden hose in the tank until it lies on the bottom, then suck on the other end to start syphoning the fuel out of the tank. Be careful not to get a mouthful of the stuff :-)
If the container is full just stop syphoning by closing the hose by bending it or pressing on it. This way you also can move around the end of the hose without the fuel going back in the tank.
Continue until most if nog all of the fuel is out of the tank, top off with diesel oil, then start motor.
If you don't want to syphon it off the classic way, go to a garage and get yourself an electric pump to get fuel out of a tank.