James,
Spray some black paint on top of your filler to use as a guide coat, then get a straight bit of wood with some sandpaper round it and work along it with that, any low spots will show up as still being black.
Depending on the type of curve on the tailgate you have to follow the curve and not go along it as this will result in flat spots, i cant remember what its like at the minute but if it is a compound curve you will have to work diagonally, that is if it curves both across the panel and vertically.
You will be best removing your lights and bottom trim if you have not already done so, as you have to smooth it into the rest of the panel.
Let me know if you need any more help.
Mat.