Woo_add_checkout_field.php

<?php
/*
*Add WooCommerce checkout Custom shipping field
*/

add_filter( 'woocommerce_checkout_fields' , 'ci_custom_override_checkout_fields' );

// Our hooked in function - $fields is passed via the filter!
function ci_custom_override_checkout_fields( $fields ) {
     $fields['shipping']['shipping_fax'] = array(
        'label'     => __('Fax', 'woocommerce'),
    'placeholder'   => _x('fax', 'placeholder', 'woocommerce'),
    'required'  => true,
    'class'     => array('form-row-wide'),
    'clear'     => true
     );

     return $fields;
}



/**
 * Process the checkout
 */
add_action('woocommerce_checkout_process', 'ci_custom_checkout_field_process');
 
function ci_custom_checkout_field_process() {
    // Check if set, if its not set add an error.
    if ( ! $_POST['shipping_fax'] )
        wc_add_notice( __( 'Please enter shipping fax.' ), 'error' );
}


/**
 * Update the order meta with field value
 */
add_action( 'woocommerce_checkout_update_order_meta', 'ci_custom_checkout_field_update_order_meta' );
 
function ci_custom_checkout_field_update_order_meta( $order_id ) {
    if ( ! empty( $_POST['shipping_fax'] ) ) {
        update_post_meta( $order_id, 'My Fax Field', sanitize_text_field( $_POST['shipping_fax'] ) );
    }
}


/**
 * Display field value on the admin order edit page
 */
add_action( 'woocommerce_admin_order_data_after_shipping_address', 'ci_custom_checkout_field_display_admin_order_meta', 10, 1 );

function ci_custom_checkout_field_display_admin_order_meta($order){
    echo '<p><strong>'.__('Custom Fax Shipping Field').':</strong> ' . get_post_meta( $order->id, 'Custom Fax Field', true ) . '</p>';
}