Very low in the mouth for very low register, very high in the mouth for the high register. As just stated in 7f, it is important to shape the required syllable in the mouth. The high register TEE creates a shape with small cavity, the low register DOO places the tongue very low in the mouth. Make sure you rest often. Too much pressure on the lip stops circulation.
Recovery is important to avoid scar tissue.
Rest for as long as is needed so that when you play again you feel fresh after your warm-up. Lips are a combination of many many muscles. Each muscle has a job to do. Trumpet players put great demands on these muscles and sometimes treat them very badly. For a 'fit' athlete to train or race without 'warming up', stretching and 'coaxing' the limbs into a preparatory state would be foolhardy.