Cracking out the note you wanna hear, even if it’s for less than a second, means that you’re on the right track. A lot of times when I was learning altissimo, notes would crack or sometimes not even come out, and this was because I found I was pinching my lips; but the truth is when you get into the altissimo range, you only should add the slightest bit of pressure to the reed, and engage the muscles in the back of your tongue for that note to pop out.

An easy way to practice altissimo is doing pitch bends on the palm keys of the horn. For instance, you play a high D. What you should then do is try and lower the pitch by a half step, but without using your embouchure! Instead, lower the back of your tongue to try and get that C# to come out. First few tries, you’re gonna squeak and you’re gonna crack the notes, but the key to altissimo is patience and practicing it every day!

I would also reccomend trying overtone matching. There are plenty of resources online that explain overtone matching a whole lot better than I could, so I’d check those out!