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

Steps

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