How to make a KML/KMZ file in Google Earth?

First, a KMZ is just a KML file inside a .ZIP file (despite the .kmz extension) so you can open the file in a program that opens zip files, then open the kml file in a text editor.
https://productforums.google.com/forum/#!topic/earth/x5KEg48eHOQ
Follow the link below to make a KML/KMZ file:
http://www.youtube.com/watch?v=TcVvyOc7f54
Basically, several steps:

1. "new placement" to create two locations;

2.  "ruler" -> "new path" to connect the two locations;

3.  "new folder" to create a folder and put the two locations and the path between them into; 

4.  "save place as" to save as KML file.

Then you can open the KML file to see the location data on Google Earth, or you can edit the KML file using vi or txt editor.

A very handy GPS visualizer:
http://www.gpsvisualizer.com/map_input?form=googleearth