After you downloaded and installed Google earth, start the program and right click the mouse into personal places folder that you will find inside Places panel on the left of the screen. Create a tour folder from the add menu, in which you will save your tracks and waypoints.
After renaming the folder and eventually, having inserted a description of the route, taking care to select the folder you just created in your personal places, press the Add Path on the toolbar at the top. Enter here a title and a short description and then, without closing the path pop-up window, bring the cursor at the starting point of your route. To move on the map, use the cursor in the upper right corner without pressing the mouse buttons otherwise Google earth will receive the input to start drawing your track. Once you find the starting point, draw the track by pressing the left button of the mouse, trying to be pretty accurate. When you are finished, in the opened window you can change the style of the line that appears on the map to choose the color and thickness. Now you can close the path window and with the Add placeholder button you create any waypoints you want: starting point, arrival and other places that might be useful (fountain, deviation ...), also including here the title and description and possibly changing the style of the placeholder. At this point you are done and you can save your route. To do this, select your folder in the Places panel and, pressing the right button, select Save Place As. Personally I prefer to save the route in .kml format as it is the one used by Google maps to create their own maps. To convert your kml file to gpx in order to upload it on your device you can use a free application directly online, on GPSvisualizer.
For those who want to create TCX files (Garmin devices), I recommend using TCXconverter, a free program where you just import previously created KML file to export it straightway in TCX format.
For users who use outdoor GPS devices more often, i recommend using a mapping software like Mapsource or CompeGPS: personally I use the excellent free software Qlandkarte: soon we will translate guides only related with this software.
Following you can see an useful video that shows what i described above... unluckly it's in Italian but still you can watch it with audio disabled and it may be useful to understand this short guide.
This guide has been realized with Google Earth 5.0 version. Actually version 7.0.2 is available to download: it is faster and has many tour features that simplify your life. Anyway this guide can be still used as basis for creating tracks with google earth.