As a remedial tool, I like to start the wind blowing and then slowly introduce the buzz into an already moving wind column. This likely will not sound all that great. But bear in mind, it is only an exercise. Often times the tongue will make the mouth close and then it is very difficult to reopen it at the point where the sound should begin. After a few blows, go back to normal tongue and blow. This can also help with range development as well. It all depends on the movement of air across the chops. No air, no buzz. Also when you master this, the attending pressure used when attempting high notes will not seem all that necessary. You may fatigue out on this in a short time, it doesn't mean that it is not working, but rather you are using muscles that are pretty green. Give some systematic exercising with plenty of rest. The other enemy here is the tongue crushing the oral cavity. Open vowels such as OO or UU will get the tongue in the lower portion of the cavity and keep it from blocking the air column. Lots to consider.
BMB F tuba 445s
BMB CC (BAT) 865s
Mack Euphonium 1150s
Wessex F Cimbasso