Inherit custom master pages to subsites in Sharepoint 2010

 Open up your Sharepoint site

• Go to Site Actions > Site Settings
• First check the following
  Go to Site Collection Administration > Site Collection Features
• Check if the `Sharepoint Server Publishing Infrastructure` is active. If it isn't you should activate it, because Sharepoint Server Publishing depends on it

 

•  Open Manage Site Features  

 

• Go to Look and Feel > Master Page
• Select the custom master from the dropdown menu
• Check the "Reset all subsites to inherit this site master page setting" checkboxes

 

• Now all subsites will use the master page