Ok I will try to explain this from memory but I'll probably leave something out, so hopefully you'll get the intent.
Go to Garmin's website and download the free Basecamp program here.:
BaseCamp
| Garmin | United States
Make sure you have saved the "file name".GPX file on your computer in a folder that you can easily find again. Plug in your Garmin to your computer via usb cable. Open the Basecamp program and open the "file name" .GPX file. Click on it and it should show on your screen. Then select "export" in the device menu to load the course file to your device. It works best for me not to have a background map overlay because it makes it harder to see the course with all the confusion. The basic GPS should show major hwys and major dirt roads already. I have attached the 2017 Baja 500 course file from SCORE to make it easier for you.