1. Open the 'Active Directory Site and Services' MMC (Located withing the 'Administrative Tools' section of the Control Panel
2. Expand the 'Sites' folder on the left
3. Expand the site that you want to alter, it may be 'Default-First-Site-Name' if you haven't changed it
4. Expand the 'Servers' folder
5. Expand the Server you wish to configure
6. Right click on the 'NTDS Settings' under the server you wish to configure and select 'Properties'
7. In the 'General' tab of that properties dialog box, tick the 'Global Catalog' option
8. Click 'Apply'
9. Click 'OK'

That's it, after replication, you will have an additional Global Catalog Server.  To stop a server being a Global Catalog Server, just untick the box explained in item 7 above instead of ticking it.