A better way instead of asking her if she wants to learn how to give great head is this:

Casually mention that most girls don't know how to give good head/blowjobs.
She'll say 'What do you mean' probably
You go 'Well they're not doing it right'........Here let me show you....then do the demo below (Using your thumb)
What this does is put a nice frame around it because now she's invested.

It's all about the teasing......the guy should be unsure if she's really gonna take it in her mouth.
That means she should first lick the cock, then kissing the shaft, and head of the cock, etc.
Then a quick sucking of the head.......and back out........push pull...
And sucking on the balls at some point.
Finally, she should take as much of the cock as she can in her mouth.
And then just in and out, back to kissing/licking, taking it all in her mouth, alternate, until the guy comes.
There's a few girls on pornhub that do it right.
NEVER spit on the cock....that's just disrespectful....the mouth has plenty of saliva already...no extra lubrication needed.
You can show her by doing all this to your thumb.

z