Pan the live aerial map to your spot, draw the racing line, add it to the library, then download a ready-to-run cab. The cab stays offline; this tool just builds it.
Drag the map so the yellow box covers the track area. Zoom changes the bake size.
In Draw line mode, click around the loop in order; it auto-closes. Switch back to Pan map to reposition.
Full offline game with every library map baked in. Copy that one file onto the cab to replace index.html. Nothing else to edit.