There is a example and information on the hook here. Throw the hook into your custom module and it should work. The below example includes a version check that you likely won't need.
function civimonitor_civicrm_permission(&$permissions) {
$version = CRM_Utils_System::version();
if (version_compare($version, '4.6.1') >= 0) {
$permissions += array(
'access CiviMonitor' => array(
ts('Access CiviMonitor', array('domain' => 'com.aghstrategies.civimonitor')),
ts('Grants the necessary API permissions for a monitoring user without Administer CiviCRM', array('domain' => 'com.aghstrategies.civimonitor')),
),
);
}
else {
$permissions += array(
'access CiviMonitor' => ts('Access CiviMonitor', array('domain' => 'com.aghstrategies.civimonitor')),
);
}
}