On the bottom of the fuel sending, there is a round nut that screws onto the rod that holds the unit together. Clean the nut (favorite toothbrush) so you can see what you have. The nut is slotted, but it also has the rod protruding through the nut so a conventional screw driver won't work.

If you have a dremel, grind the outside of a small screw driver, so that its sides are parallel and will fit inside the circular bottom. Then grind out the center of the screwdriver blade so that it fits into the slot on the nut. You need to go far enough in, so that the blade is all the into the slot on the nut and the rod will fit into the ground down area without bottoming.

DO NOT FORCE THE SCREW DRIVER IN ! Cut it so that it easily fits, and turn counter-clockwise. Careful of the three very thin wires running legnthwise in the tube. Clean all inside and out very carefully, and then reassemble when dry. If you forgot to look at the bottom two peices and how they went together, they really only go together again in EASY fashion, if they are in correct order and facing the right way. Screw all together.