All these responses are great and they might help you one way or an other. Now This is my way. Let's focus for a moment on Low Bb only. Try to sustain the note for a bit. Doesn't matter for now if you have to descend a scale to get there. While sustaining that note try to retain a mental image of how your oral cavity is shaped. Put the horn on the side and try to sing that note, imagining you are doing it through the mouthpiece (not as a you normally sing). Get back to the horn and literally try to voice that note. Please notice the embouchure shouldn't be tight, but don't relax too much either. Try to keep the sides of you face and the muscles of your jaw steady (for now). Have fun and I hope it helps at least a little bit. Ahhh!!! Please notice that even when you nail that low Bb, if you fail to keep a steady embouchure the next B, C might fail or sound weird when ascending chromatically. Help the pitch voicing. Literally sing those notes. You will find your sweet spot if they sound too harsh or too sweet for your taste.