Creating a level for Synergy is like creating a level for Half-Life 2 (singleplayer).
However, you must keep in mind that there will be multiple players running around in your level!
Players now spawn at the "info_player_coop" entities - which have been given certain features, for Synergy.
If you need to close doors behind players, or have an elevator usable once in the map, make sure you use the "info_spawn_manager" entity.
With this entity, you can move all living players to a specific location (often times the activator of the event, to keep all players together),
teleport everyone in the game instantly to a specific location and update the spawn locations (enable/disable info_player_coop entities for you).
Be sure to make use of the trigger_coop entity as well; this trigger is an extended version of a trigger_multiple and can count players and take use of a special HUD element.
Should your map require games or mods other than "hl2", you must create a Game Include Data file (.inc) for your map.
These files exist in the "maps" directory with the ".inc" extension.
They are used to tell Synergy what games are needed for each level.
The format of these files is simple:
[game2] [game3] [game4]
"game1" should always be "hl2" (Half-Life 2).
For a full list of valid default game tags, see this page.
NOTE: these files are automatically downloaded to clients when they connect to a server running the map that uses this file.