Mark the area you want to keep white with a pencil. Make light marks 1/8-inch away from the curved area that you will be painting. This space will allow the masking fluid a little leeway to bleed outward.
Dip the liner brush into the masking fluid.
Wipe the excess fluid on the edge of the bottle.
Hold the brush between the thumb and middle finger with your index finger resting on top of the ferrule (the metal band above the brush hairs). This will hold will stabilize the brush as you move around the curves of your marked area.
Place the bush on your pencil line and drag it along the curves using your whole hand for movement. Using your fingers can give you sharp lines instead of curves.