Check that the eyedropper is clean and dry. If necessary, remove the rubber bulb, wash the eyedropper and allow the bulb and glass tube to air dry. Reassemble the parts of the eyedropper before proceeding.
Pour water into a measuring cylinder with 1 ml markings. Add enough water so that the distance between the water level and the opening of the cylinder is about half the length of the eyedropper's glass tube. Note the initial water level in the measuring cylinder, by reading from the graduated scale on the side of the measuring cylinder.
Use the eyedropper to draw water from the measuring cylinder until the level in the cylinder drops to 1 ml less than the initial water level. If you accidentally extract more than 1 ml of water, gently squeeze the rubber bulb and release drops of water from the eyedropper back into the cylinder. Continue to release drops of water from the eyedropper, until the measuring cylinder water level is exactly 1 ml below the initial water level.
Hold the eyedropper with the dropper end pointing downward. Use the permanent marker to mark the water level in the eyedropper.
Empty the water from the eyedropper and allow it to dry. The eyedropper is now calibrated to serve as a 1 ml syringe.