Remove the trumpet mouthpiece if it is attached by twisting counterclockwise and pulling up on the mouthpiece.
Remove the three valves by twisting the valve caps in a counterclockwise direction. This step is crucial as the removal of the slide will create pressure inside the instrument. Removing the valves allows air to escape and avoids a pressure buildup.
Pour liquid soap down the lead-pipe. Turn the instrument three to five times so that the soap coats the inside of the tubing. Place the trumpet in a bucket of warm water and attempt to pull out on the slide. You may need to wiggle the slide slightly toward the top and bottom of the instrument to free it from the trumpet tubing.
Remove all of the slides from the trumpet, including the valves. You should not place the valves in water since they will rust easily.
Soak the slide and trumpet in a solution of warm water mixed with 2 tablespoons of liquid soap.
Push the trumpet snake into one end of the trumpet tubing and push it all the way through the horn. Pull out the snake as soon as it reaches the other end. Repeat this process with all of the slides.
Clean the trumpet valve casings with a valve brush while the trumpet is submerged. Scrub gently using a straight up-and-down motion with the brush.
Push the valve brush through the holes in each valve to remove any debris and grime that has built up over time.
Dry the trumpet thoroughly with a dry cloth. Allow the trumpet to sit out with the bell facing downward to allow all the remaining water to drain from the horn.
Apply seven to eight drops of oil to each valve and replace the valves in the horn. Screw the valve caps back into place by turning clockwise.
Polish the exterior of the instrument with a polishing cloth. Use light circles to gently polish the instrument until you have removed all of the water spots.