Setting up Synergy
In order to map for Synergy, you first have to set up the SDK.
Setting up Source
Go to the tools section in steam and scroll to the bottom until you find Source SDK. Install it, the first time you run it, you have to wait for it to finish configuring files. Next you need to download Synergy and run it at least once so it makes the proper files for source. Go to your steam\steamapps\<SteamAccount>\sourcesdk\bin\source2007\bin and find the GameConfig.txt file. Open it with notepad and scroll down to the bottom. Copy and paste the code found [here] with the other game definitions. Be sure to place it within the brackets of the "Games" key group. Afterwards if you already know how to use Hammer, you can start modding for Synergy.