Attach a windscreen to the microphone and then attach the microphone to the stand.
Place the microphone stand about 12 to 16 inches from the bell of the trumpet.
Instruct the trumpet player to aim the bell slightly above and to one side of the microphone.
Hook the microphone into the microphone input on your computer. Most computers have an input for microphones and output for headphones. The mic input has an image that looks like a small microphone.
Start your audio recording program. If you don't have one, you can download Audacity for free. This program will enable you to record audio.
Monitor the audio recording and test the sound by having the trumpet player play a few notes. Monitor the audio program by recording a test file. If the sound is coming through your audio program appropriately, press the record button and begin recording the trumpet effects. (The record button is located in the playback controls of most audio programs.)