WCF RESTful custom basic authentication

Question

Hi !! I am novice to web-services and I have already existing system with WCF Restful web service which are called by external applications and returns xml. I want to implement custom basic authentication for all of them. The requirement is that on every request there will be authentication header in HTTP request that contains Base64 bit encoded username and password. I have to intercept the request, check whether it is authenticated user or not and then response. I have checked many of project and search a lot on internet but not found any single example which contains whole this scenario. Can anyone help me please? I want a step-by-step tutorial..

Thank you,
Dev