The answer involves compass use, map reading, and field notes.
When out on a trail or old road, pick a start point and take a compass bearing as far as the trail goes before turning. Measure that distance, and you've found the first leg (compass bearing & distance) of the trail. Then head to where the path turns and get a bearing facing back towards the starting point. Repeat this process for each leg. You'll also want to measure the width of the trail in two or three places on the first leg, and any other leg where the width noticeably changes.