Cacher is the code snippet organizer for pro developers

function wooc_extra_register_fields() {
	?>

	<p class="form-row form-row-first">
	<label for="reg_billing_first_name"><?php _e( 'First name', 'woocommerce' ); ?> <span class="required">*</span></label>
	<input type="text" class="input-text" name="billing_first_name" id="reg_billing_first_name" placeholder='<?php _e( 'First name', 'woocommerce' ); ?>' value="<?php if ( ! empty( $_POST['billing_first_name'] ) ) esc_attr_e( $_POST['billing_first_name'] ); ?>" />
	</p>

	<p class="form-row form-row-last">
	<label for="reg_billing_last_name"><?php _e( 'Last name', 'woocommerce' ); ?> <span class="required">*</span></label>
	<input type="text" class="input-text" name="billing_last_name" id="reg_billing_last_name" placeholder='<?php _e( 'Last name', 'woocommerce' ); ?>' value="<?php if ( ! empty( $_POST['billing_last_name'] ) ) esc_attr_e( $_POST['billing_last_name'] ); ?>" />
	</p>

	<div class="clear"></div>

	<p class="form-row form-row-first">
	<label for="reg_billing_phone"><?php _e( 'Phone', 'woocommerce' ); ?></label>
	<input type="text" class="input-text" name="billing_phone" id="reg_billing_phone" placeholder='<?php _e( 'Phone', 'woocommerce' ); ?>' value="<?php if ( ! empty( $_POST['billing_phone'] ) ) esc_attr_e( $_POST['billing_phone'] ); ?>" />
	</p>

	<?php
}

add_action( 'woocommerce_register_form_start', 'wooc_extra_register_fields' ); 
//aparece al principio, antes de mail
//para mostrar despues de password usar "woocommerce_register_form"

function wooc_validate_extra_register_fields( $username, $email, $validation_errors ) {
	if ( isset( $_POST['billing_first_name'] ) && empty( $_POST['billing_first_name'] ) ) {
		$validation_errors->add( 'billing_first_name_error', __( '<strong>Error</strong>: First name is required!', 'woocommerce' ) );
	}

	if ( isset( $_POST['billing_last_name'] ) && empty( $_POST['billing_last_name'] ) ) {
		$validation_errors->add( 'billing_last_name_error', __( '<strong>Error</strong>: Last name is required!.', 'woocommerce' ) );
	}

}

add_action( 'woocommerce_register_post', 'wooc_validate_extra_register_fields', 10, 3 );


function wooc_save_extra_register_fields( $customer_id ) {
	if ( isset( $_POST['billing_first_name'] ) ) {
		// WordPress default first name field.
		update_user_meta( $customer_id, 'first_name', sanitize_text_field( $_POST['billing_first_name'] ) );

		// WooCommerce billing first name.
		update_user_meta( $customer_id, 'billing_first_name', sanitize_text_field( $_POST['billing_first_name'] ) );
	}

	if ( isset( $_POST['billing_last_name'] ) ) {
		// WordPress default last name field.
		update_user_meta( $customer_id, 'last_name', sanitize_text_field( $_POST['billing_last_name'] ) );

		// WooCommerce billing last name.
		update_user_meta( $customer_id, 'billing_last_name', sanitize_text_field( $_POST['billing_last_name'] ) );
	}

	if ( isset( $_POST['billing_phone'] ) ) {
		// WooCommerce billing phone
		update_user_meta( $customer_id, 'billing_phone', sanitize_text_field( $_POST['billing_phone'] ) );
	}
}

add_action( 'woocommerce_created_customer', 'wooc_save_extra_register_fields' );

billing_first_name
billing_last_name
billing_company
billing_address_1
billing_address_2
billing_city
billing_postcode
billing_country
billing_state
billing_email
billing_phone