LW, I think framing it as 'I really like makeup and I'm thinking about learning how to put it on, and I'd really love it if you did this with me' is almost guaranteed not to result in her feeling insulted.

(I think it's important to note btw that 'makeup' is not a magical skill all women have from birth and you should not approach her that way; if she very rarely wears it she may not have confidence in her skill. As an adult I've basically only worn it for weddings and job interviews, and had to ask for help every time.)