- Go to http://google.com/mymaps
- Create a new map
- Add a line or shape (under search box)
- Click around, double click to end
- Click '...' to the right of 'Untitled Map'
- Export to KML - just 'Untitled Layer' and export as KML

Or...

- Go to https://umap.openstreetmap.fr/en/
- Create a map
- Draw a polyline (right hand side)
- Embed and share (left hand side)
- Download data as KML

Upload KML file here: