The bike should be held with the rear wheel off the ground, and leaning to the left at about a 45 degree angle. An assistant may be helpful for this. Turn the pedals around a couple of times to get the wheel spinning fast, then let it coast (helps to be in high gear for this.)
Look inside the smallest sprocket. The sprocket will be stationary, since the wheel is coasting, but you'll see an inner part of the freewheel that is spinning with the wheel. Drip oil onto the crack between the turning part and the stationary part. Gravity and capillary action will help work oil into the freewheel.
Repeat this a few times, until you hear a change in the sound of the clicking pawls (they'll generally get quieter when the oil reaches them.)