Next you will drop the washer into the fuel tank (with the beveled side facing up to mate with the fuel fitting bevel) and allow it to slide down the string to the bottom of the fuel tank. You can reach your finger up into the tank outlet and insure that the washer has room to fit flat against the bottom of the inside of the tank. Too large of a washer OD may not fit.

You do the same for the fuel outlet fitting.

In the photo below the fuel tank fitting end has been slid down the string, through the washer, and out of the bottom of the fuel tank.

Now you should tie off the upper end of the string to prevent it from falling into the fuel tank as you are working on the bottom end. The string will keep the fitting and washer captive until you have them secured in place.
Slide the rubber grommet up the string and into place around the fuel fitting threads and inside of the tank hole. You will need to hold onto the outlet fitting threads to keep the fitting from going back into the tank. The grommet will hit the new washer inside of the tank.

Next slide the bottom washer and nut up the string and into place on the threads of the fuel tank outlet fitting. Tighten the nut and then reattach the fuel lines on each tank when you are finished installing hte fittings.

Your fuel tank outlet fittings will not be able to fall out of the fuel tanks now while you are flying so you now have a much safer helicopter.