The first time you run Skype after installing Call Graph, the program will ask you to install it in the client. Once installed on the client, you simply make or receive a call for Call Graph to automatically start recording.
The recordings are stored in a predefined folder in My Documents, but you can change the target location as you like. Also, if for some reason you don´t want to record every conversation you have, you can disable automatic recording in the program settings.
Finally, if you're worried about memory consumption, Call Graph is very light and runs in the taskbar so you can forget about it when you are not using it.
If you still don´t use Skype, you can download it here.