This has cropped up in other books. I'd recommend a YA book called Children of the Furnace, where a quasi-literate boy teaches an illiterate boy to read. See also, Game of Thrones with Davos and Shireen.
Basically, think of your readers. You have to balance how long it will take for someone to learn realistically - without boring the pants off your audience.
Maybe one or two scenes set around lessons (without the lessons being a focus) would work, with some references to how she's getting on. Also have her practicing in private.
It's best if you wind it into the plot. Use the shared moments to explore conflict or deepen the characters. Use her spare-time practice to come up with a revelation.
Mad rat lady writer: https://www.amazon.com/Ship-Rats-Tale-Heroism-High-ebook/dp/B07DFQJ9RL

So, I have a dark fantasy story, and in it my main character has recently freed another character who has quickly grown attached to her (Because You Were Nice to Me) and wishes to join the church of which the main character is a priestess.
Anyway, the second character, who previously spent her entire life as a slave, can't read, and one of the things she needs to do to become a priestess herself is learn how. Thing is, I don't know how to go about writing the process of teaching and could use advice and ideas on the topic.
My troper wall