How to produce GPX track files
The easy way
If you own one of the late 2011 onwards Garmin eTrex units (eTrex 10, 20 or 30) you just record and save a track then plug you GPS into the USB port. Shortly afterwards your GPS will be visible as a new disk drive on the computer and your track will be there as a GPX file.
The other way
The older Garmins use their own data storage format, so you need some software which can read it and then store it in GPX form. So, if you have not yet done so, go to the GPX TrackMaker site and download a copy of their free data processing program. While there, download the operating manual as well.
You will need to get the data from your GPS into the TrackMaker program. For this you need to purchase a cable. These are currently sold on Ebay at a price of £12 - £20.
If you are very unlucky, you may have to track-down suitable driver software. Most people will get the correct drivers provided with the cable, or even find their computer will track-down the drivers without any help.