Remove the valve cap located at the top of each valve by twisting the caps in a counterclockwise motion. Pull the valves out of the valve casings and set them down on a soft towel. Make sure to pull straight up and avoid the tendency to rotate the valves.
Wipe off any excess dirt and grime from the outside of the valves. Using a valve brush, clean the two holes located in the sides of each valve. Push the valve brush through each hole and twist the brush until you remove all the grime.
Unscrew the bottom valve caps by twisting in a counterclockwise motion. Place the valve caps under cool running water and use a cotton swab to remove any grime attached to the inside of the valve caps, around the threads or at the base of the cap. Dry the valve caps and set them aside.
Brush the inside of the valve casings with the valve brush. Buff the inside of the valves using the valve brush until the valves are clean of debris. Push the tip of a thin cloth through the valves to remove any remaining moisture.
Oil the inside of the valve casings using valve oil. Make sure to apply the oil with a circular motion so that the oil spreads down the valve casing evenly. Apply oil to the valves, ensuring that the entire valve is covered in oil. Allow any excess oil to drip into the sink or onto a cloth.
Replace the bottom valve casings by screwing them back on with a clockwise motion. Replace the valves and valve caps back onto the trumpet. The valve caps are attached to the valves, so these will be replaced in the same step.