Working with a variety of horn students inevitably the embouchure comes up as a topic. An important aspect is placement on the upper lip. Speaking generally, if the mouthpiece is well placed on the upper lip any other issue can be worked out over time.
There are two ways that I feel work well to locate a good placement on the upper lip. One is to take the mouthpiece and start with it tipped up and near the nose. Then slide it down to where it stops above the upper lip and tip it down into playing position. The other method is to take a mouthpiece and pretend you are going to drink water from it then tip it up into playing position. Curiously, both methods arrive at essentially the same placement, one that will allow for production of the full range of the horn.
When this place is found, you must also keep the lower lip and jaw engaged in the embouchure, striving for fairly equal mouthpiece pressure on both lips. It takes a combination of the right jaw placement for your lips, which may or may not be thrust slightly forward, and the correct mouthpiece angle to achieve this, but it can be done.
[And yes, my embouchure in the photo is not quite a picture book embouchure, most players are slightly non-standard.]