Log in to your server with the character you wish to make a GM
Log out to the character selection screen.
Go into your SQL editor of choice (or command line) and change account.status to 255.
Log in and enjoy GM commands (typing #help will give you a list of commands)
For example, in linux step three would look something like this:
$ mysql -u my_db_username -pmysql> use eqemu;mysql> UPDATE account SET status = 255 WHERE charname = 'my_character_name';mysql> quit;
In this example,
my_db_username is the username that you set for the mysql database during server installation and
my_character_name is the name of the character you setting to GM status.
You may find the account table schema a useful reference as well.
For a list of commands you can use once you've elevated an account to GM status, see In Game Command Reference. This reference can also be found in the table of contents on the right under Reference Lists.