Siphoning diesel fuel from one tank to transfer to another is no different than siphoning any other liquid. The key is using a method where you won't get a mouthful of fuel in the process. There are fuel transfer products available to do the job quickly, but you can also siphon diesel yourself with a few simple tools.
Push one end of the clear tubing into the diesel fuel tank that you want to drain. Make sure the hose end is under the surface of the diesel fuel by blowing in the other end of the hose. Listen for a bubbling noise.
Make a loop with the hose so that is goes down to the ground, well below the level of the fuel in the tank, then back up well above the level of fuel in the tank. You may need a chair to stand on to get the hose above the fuel level.
Suck on the free end of the hose. Watch to see diesel fill come up through the hose, then down to the ground, then proceed up again. The fuel will only rise up to the level of the fuel in the tank, so it will not come out the free end or into your mouth.
Stick the free end of the hose into the transfer tank; lower the transfer tank to the ground. The fuel will then flow from the original tank into the transfer tank.
Raise the free end high again when transfer is completed to let any residual fuel flow back into the original tank.
If you have large amounts of diesel to transfer, purchase a commercially available siphon pump or fuel transfer product.