Tighten your neck and push your chin backward. Growl from the back of your throat. The sound should be rough, like gargling or clearing the throat. It may help to relax your tongue so the front of the tongue is down and the back is raised. Practice making the sound until you are comfortable.
Bring the trumpet to your mouth, and play a lower note, such as a second-line G. Growl from the back of the throat while playing the note. If you are having trouble holding the note, make the inside of your mouth smaller with an "E" vowel shape. Practice until you are comfortable, gradually attempting the sound on higher notes.
Hold the trumpet with your right hand and bring it to your mouth. Hold the plunger with your left hand. Play a growling note, and slowly cover the bell of the trumpet.
Fan the plunger up and down to make your instrument talk. Experiment with the effect to see the different sounds you can make. This method should be attempted only when you have mastered the growling trumpet.