1. Follow the instructions in this article to save your onX Markups to your computer: Exporting waypoints, tracks, and other content from Web Map.
2. Open BaseCamp, and click File > Import into 'My Collection'
3. Select the file you saved from your onX account.
4. Your onX Markups will now be saved in BaseCamp. Connect your GPS to your computer with the USB cable, and wait for BaseCamp to recognize it.
5. Select the Markups from My Collection, and drag them into the internal storage folder of your GPS under the "Devices" heading.
Your Markups are now saved onto your GPS.