They must be round and resonant, which is created by making space in the mouth and and thinking lower in the body. A lot of time when high notes are sung I see shoulders coming up and singers reaching for them. This is natural but only hinders, think low in the body, think of pushing your back into your chair as you sing the high notes or if you are standing push on the floor and engage your pelvic floor and abdominal muscles. Keep the shoulders back and the rib cage out and up
Keep the jaw relaxed, the body and tongue create the sound not the jaw.
The length of our vocal chords limit the range of notes we can sing. We can extend our range by a few semitones up and down with proper training but there is a limit. Neither upper nor lower range must be forced and as that can damage the very fragile and small vocal folds.
Try raising your eyebrows and creating a surprised look but do beware the shoulders don’t go up. Nobody must see that you are about to sing a high note.
If you are into images then when I sing a high note I like to think of casting a fishing line in the water. You wind in your reel to prepare, you pick a spot and throw your line. Or think of a Shepards crook, and follow the up, over and forwards of the top part of the crook.
Imagery is a good tool to help you sing if images work for you. Some people can’t imagine and then a purely practical approach is necessary as outlined earlier.
Exercise
Pick out one of the practice recordings that has a high passage and practice just that passage (after a vocal warm up or humming/lip trilling/ etc). Warm ups only need to be around 5-10 min long unless you have just woken up.
Use one or two tips from above per run through and see what sounds better for you.
What is easier?
