I have never been in that situation, but here’s what I would try:

• Watch french speaking (or dubbed) videos/films/series with french subtitles activated. This way, when you hear a sentence, you’ll get used to seeing it written.

• Read books using a writing style close to spoken french, allowing you to make easy connection with the oral. (Litterature would be somewhat challenging coming from a spoken-only french background). News or magazine articles with a casual writing style could do the job too (avoid Le monde or other "serious" papers)