Hold the brush under running water and dab a bit of liquid soap on it. Gently work the soap into the brush then rinse it out completely. Clean the brush in this way even if it is new and has just come out of the package. New brushes usually have sizing in the bristles to help them hold their shape during shipping. Fold a couple of layers of paper towel around the bristles and press to dry it.
Apply one or two drops of household oil to the brush. Form the brush gently with your fingers into its natural shape.
Place the brush on a white sheet of paper on its flat side. You must be able to see it against the light background.
Hold a razor blade in one hand with your wrist on the table next to the brush. With your wrist steadying your hand, quickly bring the sharp edge of the razor blade straight down through the bristles, cutting off the tiniest bit at the tip of the brush at about a 45 degree angle. How much is cut off depends entirely on the pinstriper. Some prefer to cut off 1/64 inch, some 1/32 inch, others 1/16 inch. It all depends on the artist's technique and personal preference.
Trim a few hairs off at the ferrule, or root of the brush where the bristles are bound. This will shape the belly of the brush and is not always necessary, but it helps with sharp curves. Taking many hairs out of the brush affects how the paint flows out during painting. It is better to cut too few than too many, and you can always cut more later.