(– by pedro #2465 –)
The Trail Index will give you a PDF for each trail with description, etc. and a link to download a track. You will need a PDF reader like Adobe to open this file. Give it a read, check the cautions, etc.
Follow the link in the PDF file to get the GPX file as a ZIP file. Unzip it. The GPX file is a GPS track which shows you the path the trail follows.
To upload that to your GPS you need software to match the GPS. If you are using Garmin then Basecamp is a free download from the Garmin web site and it can open the GPX file you can click to send it to your GPS.
Note not all Nuvi models will accept uploads so you might want to check that your unit is capable before you download software and get it all set up.