In reply to captain paranoia:
Garmin BaseCamp is the weapon of choice: https://www.garmin.com/en-GB/software/basecamp/
It's not the worlds most beautifully designed, modern, slick desktop application. You get used to it. Installs in 2 mins, plug your Garmin eTrex into PC ... the eTrex will appear in context in the Garmin application side bar. (it can take a minute to appear - just wait)
There after, download the gpx file(s) from haroldstreet and import them into BaseCamp. File -> Import Into My Collection. That action is going to load the way points into Garmin Basecamp on your PC only. Final step is to push them from your PC to your Garmin eTrex. With waypoints selected - Device -> Send to Device -> Send Selection to device.
Unplug eTrex from PC. Check for way points.
Garmin Basecamp will let you make your own trails. I recommend downloading the excellent and free openMTBmap map to do so in UK. https://openmtbmap.org/download/odbl/
Cheers.