Where do I insert this code?
Before any code that reads from any of those contexts being cleaned, IOW, in the begining of your PHP code.

if (get_magic_quotes_gpc()) {
  if (!empty($_GET))    remove_magic_quotes($_GET);
  if (!empty($_POST))   remove_magic_quotes($_POST);
  if (!empty($_COOKIE)) remove_magic_quotes($_COOKIE);
  if (!empty($_REQUEST)) remove_magic_quotes($_REQUEST);
  if (!empty($_SESSION)) remove_magic_quotes($_SESSION);
  ini_set('magic_quotes_gpc', 0);
}
set_magic_quotes_runtime(0);
 
function remove_magic_quotes(&$array) {
  foreach (array_keys($array) as $key) {
    if (is_array($array[$key])) {
      remove_magic_quotes($array[$key]);
    }else {
      $array[$key] = stripslashes($array[$key]);
    }
  } 
}

Or you could do it like this:

if (get_magic_quotes_gpc()) 
{
    foreach (array('GET', 'POST', 'COOKIE', 'REQUEST', 'SESSION') as $global)
    {
        $global = "_$global";
        empty($$global) || $$global = array_map('safe_stripslashes', $$global)
    }
    ini_set('magic_quotes_gpc', 0);
}
 
set_magic_quotes_runtime(0);
 
function safe_stripslashes($element)
{
    if (is_array($element))
    {
        return array_map('safe_stripslashes', $element);
    }
    // else
 
    return stripslashes($element);
}