Using Wave Editor Mode
The wave editor is a developer feature that was disabled in the final release of the game. Thyme re-enables this feature by restoring the parsing of the command line parameter -useWaveEditor option.
Assuming you have Thyme working, you can enable the wave editor by creating a shortcut to the launchthyme binary and adding the wave editor argument to it. Something like the following in the target field:
"C:\Program Files\EA\Command and Conquer Generals\launchthyme.exe" -useWaveEditor
This will enable hotkeys which control the editor.
The editor is controlled by hotkeys that are used within the actual game. The workflow would be to create a map with the worldbuilder first, then load it in game to add the waves. The waves are saved as a .wak file alongside the map file. The editor keys function as follows: F5 - Enters/Exits the editing mode which enables/disables the other keys. F6 - Allows you to place the waves / wave types of your choosing. F7 - Saves the waves you've placed to a .wak file. F8 - Loads the existing wak file of the map, removing any unsaved changes. Insert: Choose from the five available wave types before you place them. Delete: Deletes the previously placed wave, you cannot delete long waves or existing waves.
The process generally goes as follows. First press F5 to enter editor mode. Then press F6 to enter "add start" mode and lay down the length of the wave guided by a white waypoint line. Once you've reached the desired or the end of the allowable length of the selected wave type (waypoint disappears once hit), press F6 again to enter 'add end' to place the wave. Press Insert to change the wave type being placed and Delete if you place one incorrectly. This requires precision and patience to do this multiple times for long coastal areas.
Once happy with the results, press F7 to save the changes to a .wak file next to the map file with the same filename as the map save for the extention being different. Finally press F5 to return to the normal game behaviour.
The map you are wanting to add waves to must exist in the Maps folder in your documents folder, not in the Maps folder under Data in the game directory. On modern version of windows this is normally C:\Users\%username%\Documents\Documents\Command and Conquer Generals Zero Hour Data\Maps. The editor will not write .wak files for maps in other locations.