WooCommerce: Adding multiple custom fields to the order notes area in the checkout page

functions.php

/* 1. Adds a custom field. NB. I am using some Norwegian words in the below text.
 * 2. Then adds a validate error message if person does not fill out the field. 
 * 3. Then adds the custom field to the order page. 
 https://businessbloomer.com/woocommerce-add-custom-checkout-field-php/
 https://businessbloomer.com/woocommerce-add-shipping-phone-checkout/
 https://docs.woocommerce.com/document/tutorial-customising-checkout-fields-using-actions-and-filters/#section-6
 */

add_action( 'woocommerce_before_order_notes', 'my_custom_checkout_field' );
function my_custom_checkout_field( $checkout ) {

    woocommerce_form_field( 'billing_year', array(
        'type'          => 'checkbox',
	'required' 	=> 'true',
        'class'         => array('year-class form-row-wide'),
        'label'         => __('I am over 18.'),
        'placeholder'   => __(''),
        ), $checkout->get_value( 'year' ));


	woocommerce_form_field( 'forening', array(
        'type'          => 'text',
	'required' 	=> 'true',
        'class'         => array('forening-class form-row-wide'),
        'label'         => __('Which sportsclub/organization or class do you belong to?'),
        ), $checkout->get_value( 'forening' ));

	
	woocommerce_form_field( 'antall_personer', array(
        'type'          => 'text',
	'required'    	=> 'true',
        'class'         => array('antall-personer-class form-row-wide'),
        'label'         => __('How many will sell these products?'),
	'priority'	=> 28,
        ), $checkout->get_value( 'antall_personer' ));


	woocommerce_form_field( 'kontaktperson', array(
        'type'          => 'select',
	'required' 	=> 'true',
        'class'         => array('kontaktperson-class form-row-wide'),
        'label'         => __('Contact from our company'),
	'options'  	=> array( // options for <select> or <input type="radio" />
	 ''    		=> 'Please select', // empty values means that field is not selected
			'ingen_kontakt_person'	=> 'No contact person', // 'value'=>'Name'
			            'Regine'	=> 'Employee 1',
			             'mei_mei'	=> 'Employee 2'
		)
        ), $checkout->get_value( 'kontaktperson' ));
}


/**
 * 2. Process the checkout - We then need to validate the field. If someone does not fill out the field they will get an error message.
 */

add_action('woocommerce_checkout_process', 'my_custom_checkout_field_process');
function my_custom_checkout_field_process() {
    // Check if set, if its not set add an error.
    if ( ! $_POST['year'] )
        wc_add_notice( __( 'Fyll ut: Bekreft alder' ), 'error' );
	if ( ! $_POST['forening'] )
        wc_add_notice( __( 'Fyll ut: Hvilken idrettslag/ forening/ klasse?' ), 'error' );
	if ( ! $_POST['antall_personer'] )
        wc_add_notice( __( 'Fyll ut: Antall personer som skal selge?' ), 'error' );
}


/**
 * 3. Display field value on the order edit page.
 */
		
add_action( 'woocommerce_admin_order_data_after_billing_address', 'my_custom_checkout_field_display_admin_order_meta', 10, 1 );
function my_custom_checkout_field_display_admin_order_meta($order){
    	echo '<p><strong>'.__('Over 18').':</strong> ' . get_post_meta( $order->id, 'Over 18', true ) . '</p>';
	echo '<p><strong>'.__('Foreningen').':</strong> ' . get_post_meta( $order->id, 'Foreningen', true ) . '</p>';
	echo '<p><strong>'.__('Antall personer').':</strong> ' . get_post_meta( $order->id, 'Antall personer', true ) . '</p>';
}

Good to hear!