Play an F below middle C on your trumpet, then remove your mouthpiece and match the pitch by buzzing on the mouthpiece. You may use a piano to match pitch as well.
Hold the low F for four beats with closed lips and the attempt to jump down to pedal C on the mouthpiece. The trick to playing low notes is to relax your embouchure, which will make the lips vibrate slower. Use a slow airstream to prevent the lips from vibrating too quickly. The correct embouchure resembles a frown.
Hold pedal C for four beats to ensure you have a solid sound. Then using all open valves, move up to E on the staff, then G, then down to E and down to middle C. Practice this exercise several times with the instrument and with the mouthpiece until it is comfortable.