SharePoint 2010 FBA : Setup custom sign in page

There are 2 ways to do it. 

Go to web application web.config file

<authentication mode="Forms">
<forms loginUrl="/_login/default.aspx" />

</authentication>

Change like this

<authentication mode="Forms">

    <forms loginUrl="~/_layouts/yourfolder/signin.aspx" />

  </authentication>

or

Central administration > Secuirty> Specify authentication providers > Select Web Application from right side dropdown > Extranet (zone)