Learn enough to get by . . .
...know what different words look like, that's it, and only the really important ones. It helps if they're backed by a commonly used symbol, as with stop and yield signs. Go is generally a green word. Some people don't really learn to read until later in life, finding that knowing what different syllables look to be quite sufficient -- for a while. Then they try for the professional field, one where they’re expected to spell words correctly and use proper grammar in their interoffice emails.
You don't actually set out to become functionally illiterate, like it was a goal -- things come along, life changes occur, and it just sorta happens.