Use a metronome. Set it to MM=60.
1. Do this as a breath exercise. Try to
hear the pithces in the breath sound.
2. Do it as a buzz exercise with glissando. Slide from note to note.
3. Do it with legato and centered tones.
The last bar is a so called pedal notes on the trumpet.
You should use the lower lip more active as you descend. By buzzing
these low notes you will strengthen the lower lip.