Start with watching the Signing Time videos without captions or sound. Full speed at first and then half-speed on the parts you don't understand. Hopefully your library has copies. Take an ASL class if available. This will get you words to work with. ASL actually has a different grammar and uses dimensional space, so maybe you want to learn PSE (Pigeon Signed English).
Each of you can take turns writing down and practicing words from different parts of an English-ASL dictionary. Stop speaking in normal conversation to insert an ASL word you know. You just need to fingerspell or try and pantomime the meaning of new words when you first them show each other. It takes a lot of effort to say something in a different language.
We are all deaf at different points in our lives. Some of us just have the privilege and opportunity of learning a sign language earlier than others.
