Everyone has their own way of warming up to sing. Because I’m an analytical teacher with a tendency to overthink, I need to direct myself toward more primal, spontaneous sounds to get my singing self going. Orderly sequences of scales and arpeggios are great for the heart of a practice session, but not for the first vocalizing of the day. Very often as I start to improvise on pitch patterns, I end up with pentatonic scales and riffs. After I’ve played a bit all over my range, mostly with my favorite vowels (/a/ and /o/), I’ll sing lines with words. Then I am ready to practice, if it is that time of day. However, oftentimes the warmup is all that happens, since I am in a car or walking down a street and have to move on to something else.
I am not a fan of the very popular “semi-occluded” exercises for myself, but they are helpful and comforting to many other people. Some people will do a lot of these or other specific exercises every time they warm up. Others will sing certain songs or solfeggio patterns. There is no one right warm-up for everyone. Just make sure that it’s easy, fun, and more experimental than judgemental.