I don’t do Facebook, but it’s very possible to do harmonic slurs with bad form. I did for 20+ years and still catch myself occasionally reverting back to clamping my lips in the middle top-to-bottom, which you can get away with on the staff…

But as you ascend higher, and the aperture gets smaller, clamping top-to-bottom in the middle of your lips inevitably results in your aperture collapsing (closing the aperture) and/or your top lip being flexed to the point that it doesn’t vibrate as quickly as needed to achieve the desired pitch.

By tightening the aperture from the sides and leaving our top lip relaxed, we support the top lip with the corners like tent poles holding up the roof (our top lip) from collapsing.

The Ooooohhhhh in MmmmmwwwwaaaahhhOooohh gives us the sensation of our aperture corners engaged.

For me, what works for me to get going is to focus on a big breath and releasing it in one long steady passive sigh out as I think Ooohhhh and focus on engaging,tightening the aperture as little as necessary to achieve the next pitch. Then I repeat faster and faster, focusing on steady passive air–kicking the air is a crutch and bad form and must be avoided.

I find middle C to E is actually easier for me to get going at first, as its a smaller interval and easier to start with. G to C is a bigger interval and therefore a little harder.

Important to focus on paying attention to what you’re doing–tightening from the corners, or top-to-bottom;I catch myself reverting back to old tricks occasionally.

Hope that helps; works for me. Good luck!