It may sound silly, but you really don't want to get your valves halfway disassembled before you have your oil ready to go. Take a moment before you start to get out your valve oil and open the bottle. You'll have one less thing to fumble with while your delicate and expensive to repair valves are vulnerable. If you happen to be running low on oil, I recommend Hetman No. 2:
Before disassembling your valves, hold the trumpet in your non-dominant hand at about a 45 degree angle (see photo at right). This will help insure that the valves stay in position while you oil them instead of either falling out or back into the horn.
Unscrew the valves caps on all three valves at once, then pull each valve PARTWAY out, as in the photo.
Be careful not to rotate the valves as you pull them. They must go back in the same orientation they were in before in order to work properly.
Put a few drops of oil on the smooth exposed area of each valve. Three to four drops per valve is usually about the right amount, but vary that as needed for your instrument.
You don't need to oil the spring, or the part of the valve that holds the spring. The only area that needs oil is the part that comes in contact with the valve casing (see photo).
Carefully slide the valves back into position, taking care not to spin them out of alignment as you do. On most trumpets, there is a number stamped onto the valve near the top that tells you which valve this is (1, 2, or 3). This number usually faces the mouthpiece, but on some horns it's the other way around.
DO NOT FORCE THE VALVES BACK INTO THE HORN!!
If you have any trouble getting them back into position, gently, slowly, carefully wiggle them back and forth and up and down. This will usually do the trick. Some valves are more finicky than others... just be patient.
Once the valves are in position, carefully screw the valve caps back on. They should be only "finger-tight." Remember that you will need to unscrew them again the next time you need to oil your valves!
After everything is back in place, it’s always a good idea to blow some air through the horn and make sure everything is working. If you can, play a note or two to make sure they come out. If you’ve accidentally put a valve or two in backwards, you’ll discover it now instead of during your performance!