Head Canon Time:
Sokka eventually got tired of Toph’s blind jokes.
Katara [Cheerfully.] We found a printer to make our posters! [Reveals the poster.]
Sokka: Hey, I thought designing the lost Appa poster is my job. I’ve been working all day on my Appa! [Shows his drawing, which is quite crude.]
Katara tries to hold back laughter.
Aang: Sokka, the arrow is on Appa’s head.
Sokka: This is his head.
Katara: Why are feet coming out of it?
Sokka: Those are his horns. I haven’t seen him in a while, okay?
Toph: It looks just like him to me!
Sokka: Thank you, I work really - [Remembering she is blind.] Why do you feel the need to do that?
So he decided to do something about it. One day, while visiting the Bei-Fong Academy for Metal Bending, he was practicing his drawing as his Master had instructed him to do, and then it hit him. Toph had told him that she learned to metal bend by feeling out the impurities in the refined ore. That she bends metal by feeling out the very very small parts of earth still in them, just like she can bend mud even though its part water. And his genius inventor mind went to work as he dipped his brush into the vial of ink…
Sokka had Toph crush and powder some flakes of metal, which he then mixed into a barrel of ink. Ignoring her cries of “I told you this isn’t going to work snoozles”, he began to draw with them, in broad strokes at first so she could get used to them. Once she was able to describe these blocky simple drawings, he started getting finer and finer in his art, and eventually moved on to written letters. He taught her how to read through a combination of her bending and touch, a braile of sorts that only needed to be touch for the very detailed drawings.
In the end, this allowed her to see his art, and greatly move forward in her development of the Metal Bending forces, as she could now send and receive status updates without needing someone to read or transcribe for her. Best of all, he gave her a way to see her daughter’s beauty as everyone else could, and she thanked him most for that.(even if she couldn’t care in the least about seeing what she already knew was the case)
She still kept making blind jokes at his expense.
The only day she cursed him having taught her that, is the day she received an invitation in the mail to the Wedding of Sokka Water Tribe and Suki of the Kyoshi Warriors. (because old crushes die hard…)
TL:DR; Sokka taught Toph how to read and see his drawings by adding metal to ink.