This depends on your particular setup, but you just modify your phpmyadmin config file to connect to the RDS database, and use phpmyadmin as usual.
Go to the phpmyadmin's root folder (usually /var/phpmyadmin) and copy config.sample.inc.php to config.inc.php .
Inside the file set up yout RDS connection:
/* Authentication type */
$cfg['Servers'][$i]['auth_type'] = 'cookie';
/* Server parameters */
$cfg['Servers'][$i]['host'] = '<RDS host>';
$cfg['Servers'][$i]['connect_type'] = 'tcp';
$cfg['Servers'][$i]['compress'] = false;
/* Select mysql if your server does not have mysqli */
$cfg['Servers'][$i]['extension'] = 'mysqli';
$cfg['Servers'][$i]['AllowNoPassword'] = false;
Remember that the server where you are running phpmyadmin must have access to the RDS.