If this is page by page, you do the following:

1. Create a directory called App_LocalResources
2. Name the resource file for default.aspx default.aspx.resx
3. Use the following tag to get the value Header from this file (assuming a
name of HeaderText)

<%$ Resources:HeaderText %>

as in

<h1><asp:Label runat="server" Text="<%$ Resources:HeaderText %>"
id="Label3"/></h1>

For global resources

1. Create a directory called App_GlobalResources
2. Name the file something like GlobalResources.resx (name not as important
as matching the file name to the class name in the next step)
3. Use the following tag (note the GlobalResources class name as this is the
major change from local resources)

<%$ Resources:GlobalResources, HeaderText %>

as in

<h1><asp:Label runat="server" Text="<%$ Resources:GlobalResources,
HeaderText %>" id="Label3"/></h1>

Rule: ClassName = RESX file name.

If you do this, ASP.NET can be set up to automatically localize content when
you have the proper language resource files. You will have to add a
globalization tag to the web.config.

For a great intro, find Michele Leroux Bustamante's articles on MSDN. You
can also find her blog at www.dasblonde.com.