From his wikipedia article:
https://en.wikipedia.org/wiki/Frederick_Douglass
Douglass related how he learned to read from white children in the neighborhood, and by observing the writings of the men with whom he worked. Douglass continued, secretly, to teach himself how to read and write.
How does that work??
He provides more detail in his autobiographies. At first, his Yankee mistress taught him the basics, until her husband told her to stop. Then, when playing with white boys (for awhile his job was to be the playmate of his master's young son), he asked them to tell him what they'd just learned, and thus he got their lessons second-hand.