Key points:
Add the WCF configuration to the web.config of your SharePoint site.
Configure services to start in ASP.NET compatibility mode. This ensures that they run through the ASP.NET pipeline, which is required for SharePoint.
Create a path to store the .svc files in the IIS website directory. Set it unmanaged so that SharePoint does not interfere.
Deploy the DLL for this service.
There are many details that do not fit. Read the Sahil Malik posts for complete examples and additional information on how to do all of the above, as well as additional methods such as packaging in solutions. There's also an article by Steve Fox , which is also pretty detailed.
If you want to delve into WCF, including the chapter on integrating with SharePoint, read Daniel Larson's AJAX Service Oriented Application Development on Microsoft ® Platform .