Hi SS220
This is what I do, (you do need a Google account) Sign in to your google account, top left look for icon called google apps (black dots 3 across 3 down) click on this and open "Maps", then in the search bar (top left) look for the "Menu" icon (3 parallel lines) click it and scroll down to "My maps" and click "Untitled Maps" below that you will see a prompt called "open in My Maps", click on this and you will see that there are now some drawing tools, use the search bar and type in the location you want to map. On the map use the "Draw a Line" tool to map out an area, rename the resulting Polygon to what ever you want, eg "My field map".
When you have done this you will see a prompt in the map window "upper left of screen" click on the 3 dots opposite "Add Layer", and some options will open one will be "Export to KML" you will then be prompted to Download the file, do this and you will have your .KML file
Regards
Waylander