Creating a map for Synergy is like creating a map for Half-Life 2.
However, you must keep in mind that there will be multiple players running around in your map!
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, so that none of the players will be left behind.
You can then decide to update the spawn locations by enabling/disabling the "info_player_coop" entities which you can put at any location in the map, to get a "checkpoint" effect.
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 map.
The format of these files are very simple, and are separated by spaces only:
hl2 game2 game3 game4
"hl2" should always be the first game in your ".inc" file.
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.