Reddit reviews Embouchure Builder for French Horn

Grab this, do the high attack exercises, grab any flexibility exercises, I have a Hector McDonald routine, Stamp is also great, but anything that gets you sustained high work is the way to go. Try this pressure reducing exercise: Sustain a C in the stave, slowly move the leadpipe away from from lips until you're barely holding on to that sound, then pull back in until you get a good tone again. It'll help train you to use less pressure, and when you play high, on-mouth pressure is the enemy. That takes care of the technical side

Process cues: Think lower air to go high. imagine the air dropping lower and lower as you go higher, it'll help keep the back of the throat open, and you need a lot of core support to get there anyway. Tighter lips isn't really it, it's fast air, a whistling sensation that pops you into the upper partials, meaty support in the lips and most essentially: singing brain. Tongue lift is also important, if you can whistle, as the sound goes higher the back of your tongue comes up. Same thing happens in the horn, so, something to think about.

If all else fails, work with a high specialist in person to get exact diagnostics on your present play style. tbqh, everyone has their natural limits. I'm never going to get down to double pedal C if I live to be 100, but I can get that E above the stave no drama llama ;-)

I love Joe Singer's Embouchure Builder. Great book.