• 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: