WordPress Nonce and check admin referer help needed

Just before I start please note that I am new at any type of WordPress/PHP security. Few days ago friend of mine asked me to create Theme Options for his Theme, all set and working fine, however he had few minor issues with his Theme in the past and I would like to make sure that Nonce and check_admin_referer is set properly.

Here is what I used:

http://codex.wordpress.org/Function_Reference/check_admin_referer http://codex.wordpress.org/WordPress_Nonces

And now my question is: where exactly wp_nonce_field needs to be in and where check_admin_referer?

Before the form start I have:

    if ( !empty($_POST) && check_admin_referer('crt_nonce') )
    {
$saved = false;
if (  isset( $_REQUEST['action'] ) && 'save' == $_REQUEST['action'] ) {

and just before form closes the wp_nonce_field

    <form action="" id="save" method="post">
    <?php wp_nonce_field('crt_nonce'); ?>
    </form>

There are few articles about this around, but they don’t help me at all. If anyone can and wants to assist me here I would very much appreciated. And I will do my best to follow your instructions religiously and try to avoid more questions 😉

PS Oh, so that I don’t forget 🙂 wp_nonce_field and check_admin_referer needs to be set once correct, not for each field options that I’ve created?

Thanks,
Draga