Ideally, you should have a level file (level.inf) and a script file (*any name*.scp). This is an excerpt from Haegemonia Level Planner's readme.
To install the level into Haegemonia, do the follwing:
1: Create a folder in your Haegemonia - The Solon Heritage\Game_Inf\Levels folder. Call it anything you'd like - I will call it MyLevelFolder.
2: Copy the files level.inf and (filename).scp to the MyLevelFolder directory.
3: Now, go back to the Game_Inf\Levels directory
4: Open the skirmish_levels.lst file in a text editor, for example Notepad.
5: Add a line with the path of your level (relative to the Haegemonia main folder), for example: Game_inf\Levels\MyLevelFolder
6: Do the same for multi_levels.lst if you want to use the map in multiplayer mode.
7: You are done.
It is pretty much self-explanatory. However, there are unknown constraints to the number of levels available in the lists. If your level does not appear in the Skirmish/Multiplayer levels list, try to replace an existing level's entry in skirmish_levels.lst or multi_levels.lst with your own level's line.
It is a good idea to keep backups of the list files. It makes your life easier when you want to restore the old level list.
When playing multiplayer games, only the host needs to have the map installed. When starting the game, the level file is moved to every player's PC into the Game_Inf\Levels\Level Transfer folder. (You can find the last used MP-map here)
Good Luck!
Soon there's gonna be a tool which allows you to easily compile your own map lists, and manage these.