Set once at the start of a track. Every leg below carries this forward using mid-latitude sailing.
A nautical mile is exactly 1,852 metres (1.852 km, 1.1508 statute miles, about 6,076 feet) — defined so that one nautical mile equals one minute of latitude on the Earth's surface. That's why speed in knots (nautical miles per hour) converts straight into minutes of latitude and longitude here: run 6.0 kts for 1 hour and your DR position moves 6.0 minutes along your course — no separate distance scale required, on this tool or on a real plotting sheet.
Schematic plot — north up, not to a chart scale. For teaching the shape of a DR track; plot true positions on your chart or plotting sheet.