Steps to apply custom master page to all SharePoint 2010 sub sites

Steps

1. Go to Site Actions -->Site Settings.
   
2. Click on Manage Site Features.
   
   
   
3. Activate SharePoint Server Publishing.
   
   
   
4. Go to Site Actions --> Site Settings again and under Look and Feel click on Master Page.
   
   
   
5. In Site Master Page click on the drop down and select your custom master page.
   
   
   
6. Put a check in Reset all subsites to inherit this site master page setting and then Click "ok" at the bottom of the page.