Here’s a lengthy video by trumpet player Charlie Porter on how to form a brass embouchure.
I have had some disagreements with Porter in the past. I have some quibbles with some of his instructions too, but I like his recommendation to set firm the lips up before setting the mouthpiece on the lips. Two of the other steps he recommends (pulling the lips open after setting the mouthpiece and wetting the lip center with the tongue after that) I feel would risk undoing the value of firming before setting.
Watching through the video I didn’t understand if he was suggesting the embouchure aperture remains open throughout the lip vibration, so I asked him about it. He was kind enough to take the time to clarify for me.
Of course the lips rapidly close and open during vibration. That’s not the point…I’m not arguing that they they never close briefly, per each vibration occurring…the point is that players are often way too tight and begin with closed lips and press them together to the point of distorting the vibration.
It’s a rather long video, but take look at it if you’re interested in different thoughts about setting the embouchure formation for playing.