Take it slow and easy. Do not rush to play a lot of different notes.

Rewatch the “Leadpipe to Trumpet Transition” video.

It is not surprising that your notes are not spot-on at first. You played the leadpipe with a wide aperture that was probably more suited to the trumpet’s pedal tones.

You can noodle around on the horn for fun, seeing what notes you can play. Warning: This could lead to bad habits and injury. My advice is to be more systematic and wait to learn the other notes by gradually learning to change shape.

Follow WindWords exactly as prescribed.

Here is a beginner exercise that could help you. It sounds like your first note has landed on the low C, which is the C below the staff (stave). Listen to Greg playing this note. You play it and try to match his sound. Do this several times.
Repeat this exercise several times. Focus on playing with passive air and keeping the airflow steady.

Exercise 2. Set a metronome at 80 bpm. Play the C for four counts and rest for beats. Play C four times with a 4 beat rest between each note. Repeat several times. Use the Singing C drone while doing this exercise. Continue this exercise for many days. It can be used as a simple beginner warm-up. If you can, practice the exercise for short periods many times a day.

EXercise 3. Play Greg’s Sing C exercise, Step 1.

Take your time. You may spend 2 to 5 days on these exercises. Continue to practice the backswing and release and all the beginning exercises (backswing, release, etc…) along with the leadpipe exercises. When this is consistent and easy, move on to step 2. I can’t say how long you need to play an exercise before moving to the next. You go on to the next exercise when the current exercise can be played consistently and easily.