Open the editor and start with a GPX trace or create a new track. Give it a recognisable name, check its position on the map, and refine the points that need attention.
Once the geometry is ready, define start/finish and direction, then work through sectors and corner boundaries. Sign in with Google to keep the track in My Tracks.
Start with a simple circuit so you can learn the controls before tackling a complicated layout. The How To page walks through the main editing workflow.