The sound begins its life using the tongue behind the top teeth by saying one of the following syllables "Da, Day, Dee, Du, - Ta, Tay, Tee, Tu, dependant upon the amount of accent that you wish the note to start with. The tongue then having done most of its work, not all but most, then drops lower in the mouth and at the same time the diaphragm (stomach) pushes air towards the lips at a constant rate to vibrate them at the desired pitch. The more air pressure from the diaphragm plus the tongue will give you the desired accent, pitch and volume of the sound. Vibrating the lips at 440 times a second (hertz) would produce a concert pitch "a". The sound that you should now produce should have a full sound with no wasted air producing a hiss to accompany it. If a hiss is present then either the embouchure needs to be adjusted or the air pressure from the stomach needs to be controlled better.
It is up to the performer to really listen to his or her sound and scrutinise it for the above impurities. In all the above, the tongue position is critical. If the tongue is too low in the mouth then the note will try to drop in pitch to the next one playable, with the valve combination you are using. If the tongue is too high in the mouth then the note can become thin and strident in colour, and not particularly good to listen too. Therefore the tongue position should be as low as possible in the mouth for the given pitch, supported by a constant stream of air from the stomach. Once a good pure tone is achieved, adding vibrato will carry the sound and make it more mellow which is good for slow lyrical work. However for orchestral work this vibrato should be used sparingly.