Before you do this, please see this post if you get any error message.
How to apply custom master page and theme to all sub sites in SharePoint 2010?
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. If your not sure how to create custom master page, please see : http://virtualizesharepoint.com/category/sharepoint-2010-branding/
6. Put a check in Reset all subsites to inherit this site master page setting and click OK in the bottom of the page