Long explanation. It may be covered in a video. I've never looked. My DH calls this the 'Apocalypse Now' approach.
Sit on the floor with the cat wedged between your knees/thighs facing outward. (Important because any attack (teeth and/or claws are directed away from you.) Put bottle in opposite hand – i.e. if it is the cat's right eye, hold the bottle in your left hand. Bring hand holding the bottle in from behind the cat's head. The cat will look up. Grab the cat's head with your other hand (on the side of the affected eye), with thumb at base of jaw, index finder around the whiskers above the eye, remainder of fingers below the eye.
If the cat keeps its eye opened, lucky you, instil the drop.
If the cat closes its eye, you can open it by spreading your index finger (above) and your middle finger (below) the eye.
It's late night, and I'm very tired. The above might make any sense at all. I hope it does. You don't want to know how many times I placed my fingers on the side of my head so that I could describe the process accurately. Note to self: remember to close the curtains at night before making weird faces with my fingers for anyone walking by outside can see.