When musician speak of doubling, they are not speaking of playing bridge. Doubling, as most of you know is the act of playing more than one instrument. For a woodwind player, this is much easier for the mouthpiece, read and fingering are much the same among the woodwinds. Obviously the flute is one exception. For trumpet players we have a similar situation in that the mouthpiece and fingering among the members of the trumpet/flugel horn/cornet family are again very similar. But what I will be addressing here is the possibility of doubling on trombone; although from the same family of brass instruments with different fingerings and in this case slide positions.
Here is how you can learn to double on a trombone when you are a trumpet player.
Important fact #1
Both the trumpet and trombone are basic, overtone (harmonic) playing instruments.
Important fact #2
Both use cup mouthpieces to create sound.
Important fact #3
At this point in your life you already know how to play trumpet and this will be of great value when learning to make the switch back and forth.
Instructional point #1
You have to have a working trombone. If you instrument has slide issues, this will make things more difficult. If it has holes in it where holes should not be, you will have problems.
Instructional point #2
Download and print out these PDF documents.
Instructional point #3
Follow the instructions on the document.
Trumpet/Trombone Doubling Instructions