As you may know, in WSS 3.0, we don’t have the option through the UI to set a custom master page file. In MOSS 2007, we have this "Master Page" action link that lets us select a different master page then the one initially configured for the site. So what are our options, here are a couple:
Using SharePoint Designer
1. Open your WSS 3.0 Site and Expand the "_catalogs" node then the "masterpage" node.
2. Select your custom master page, then Right Click on it and choose "Set as Default Master Page"
1. If you haven’t installed and played around with Powershell, I highly recommend doing that as soon as you can. You will have fun 🙂
2. So, you’ve installed Powershell and now you are an expert. With Powershell 2.0 you have the option to use Powershell Integrated Scripting Environment or the ISE. Go ahead and open that. Copy and Paste the following code snippet to set a custom master page file for your WSS 3.0 site