When figuring out both how much pressure to use and how fast you should move, you need to listen to her body and see what she reacts to.
In Frömming, Urte Undine; Köhn, Steffen; Fox, Samantha; Terry, Mike eds.
How fast should I be? She started shaking to orgasm grabbing my head and grinding her pussy.
You can learn a lot from mature videos and take notes.
Check out the Wikipedia pages for both the and Youtuber Stevie gives pussy eating tips as part of her Lesbian Sex Ed series.
There are a bunch of ways to finger her while eating her out, with some being easy to do, while others will feel quite awkward.