From the faq.
How do I add fuel to my Zippo lighter?
Remove the inside unit from the case. Lift the corner of the felt pad on the bottom of the insert to reveal the packing material in the fuel chamber. Saturate the packing with Zippo lighter fluid. Fill slowly, taking care to never overfill. Avoid getting the lighter fluid on your skin as it can be a skin irritant. If contact with skin does occur, wash the effected area with mild soap and water.
Insert the inside unit back into the case, making sure to wipe the lighter and your hands dry before igniting. Be sure the fuel can is closed and there is no spilled fuel in the vicinity before igniting. This is a flammable fluid.
If the Zippo lighter is kept in your pocket, Zippo suggests that it be placed bottom down, especially when first re-fueled.
I suggest filling it at night and leaving it alone till next morning, that lighter fluid will burn the hell out of you. I put one in a shirt pocket after refilling, ONCE.