How to achieve Custom Forms Authentication

Folks,
In my earlier verison, I used Active Directory to authenticate users which was Custom. In the sense that, I had passed UserName and password along with a token request through datalayer to authenticate against AD. it would eventually check the DomainName\UserName, password against AD and will get authenticated.
Well, now, I have to do similar thing using Forms.
Can some one explain in detail how to I achieve this custom forms authentication?
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