Steps
  1. Go to Site Actions -->Site Settings.
  2. Click on Manage Site Features.

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  3. Activate SharePoint Server Publishing.

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  4. Go to Site Actions --> Site Settings again and under Look and Feel click on Master Page.

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  5. In Site Master Page click on the drop down and select your custom master page.

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  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.

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