add-field-to-custom-meta-data-box.php

<?php
/* Add a field to the metabox */

function jpen_custom_post_order( $post ) {
  wp_nonce_field( basename( __FILE__ ), 'jpen_custom_post_order_nonce' );
  $current_pos = get_post_meta( $post->ID, '_custom_post_order', true); ?>
  <p>Enter the position at which you would like the post to appear. For exampe, post "1" will appear first, post "2" second, and so forth.</p>
  <p><input type="number" name="pos" value="<?php echo $current_pos; ?>" /></p>
  <?php
}
?>