You save them in a file somewhere. When you renew your medical license, you'll need to submit them. You can keep a paper file, or scan them and save them on Google Drive or something similar.
Also check with your state's medical board as they may have an electronic, online system if you prefer that route. Here in Florida we have CE Broker where some CME is automatically submitted to them or we can upload our own and they then log it and keep track.