Once, we have built the Master Pages we need to assign the
Master Page to our site. This can be achieved as follows:
Step 1: On you site, click Site Actions->Site
Settings->Modify All Site Settings.
Figure 5

Step 2: When the site Settings Window displays, Click on the
Master Page under Look and Feel
Figure 6

Step 3: Select the Master Page from the drop down list.
Figure 7

Step 4: Most likely you will try to create your own SHB file
to replace the look and feel of some of the CSS class dependent objects. For
example, you might want to have a different look and feel for the Web Parts
header or may be would like to change the fonts on links. For this purpose you
might decide to assign a different CSS file. You can assign it on the same Site
Master Page assignment page.
Figure 8

If you have specified a CSS file and have not handled a
style in your CSS file then SharePoint will pick the default style assigned to the
control. I will be writing an article about the various CSS styles and what
they are used for.