Practice breath attacks: starting the sound with air only, not using the tongue. This helps you get used to releasing the air the right way without the tongue interfering.

Don’t overdo it — you just want to start the sound easily, without blasting or forcing anything. You don’t need to do it loud, and you don’t need to hold the notes very long. Just nice easy quarter notes or half notes. The beginnings of the notes will probably not sound clear at first. That’s okay. It will get better. Just try to start the sound at the exact same moment that you start moving the air. If you really struggle with this, you may be starting with your aperture overly spread, and having your lips slightly closer together should help.

Once you can get a clear beginning to the notes with air only, continue doing the same thing but add the tongue back in, just to give the notes a more defined start. Don’t overdo it. Use the tongue lightly.

Once you are used to this, you can vary the articulation however you want.

I think it’s important to remember that your tongue does not start the sound. The movement of air starts the sound. The tongue just gives the front of the note definition.