MySQL Order by show specific data at the top - mysql

Question

I have a below data in my activity table. I want to show those records at the top whose followup_date is today onwards in ascending order, after that those records whose followup_date is past date in ascending order and after that those records whose followup_date is null.
DROP TABLE IF EXISTS `activity`;
CREATE TABLE IF NOT EXISTS `activity` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`type_id` int(11) NOT NULL,
`followup_date` date DEFAULT NULL,
`created` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP,
PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=9 DEFAULT CHARSET=utf8;
--
-- Dumping data for table `activity`
--
INSERT INTO `activity` (`id`, `type_id`, `followup_date`, `created`) VALUES
(1, 1, '2022-03-22', '2022-03-24 18:51:23'),
(2, 1, '2022-03-23', '2022-03-24 18:51:23'),
(3, 1, '2022-03-24', '2022-03-24 18:51:58'),
(4, 1, '2022-03-25', '2022-03-24 18:51:58'),
(5, 1, '2022-03-26', '2022-03-24 18:52:21'),
(6, 1, '2022-03-13', '2022-03-24 18:52:21'),
(7, 1, NULL, '2022-03-24 18:54:15'),
(8, 1, NULL, '2022-03-24 18:54:15');
I tried using below query but could not understand how would i use ORDER BY CASE statement to get the result mentioned below.
SELECT * FROM `activity` ORDER BY CASE WHEN followup_date IS NULL THEN 2 WHEN followup_date >= '2022-03-24' THEN 1 END ASC
Current Output:
Expected Output
What changes i will need to make in above query to get Expected Output

Answer 1

I moved the expression into the select-list so we could see it in the result, but you may keep it in the ORDER BY clause:
SELECT CASE WHEN followup_date IS NULL THEN 2
WHEN followup_date < '2022-03-24' THEN 1
ELSE 0 END AS sort_bucket,
id, followup_date
FROM `activity`
ORDER BY sort_bucket ASC, followup_date ASC
Output:
+-------------+----+---------------+
| sort_bucket | id | followup_date |
+-------------+----+---------------+
| 0 | 3 | 2022-03-24 |
| 0 | 4 | 2022-03-25 |
| 0 | 5 | 2022-03-26 |
| 1 | 6 | 2022-03-13 |
| 1 | 1 | 2022-03-22 |
| 1 | 2 | 2022-03-23 |
| 2 | 7 | NULL |
| 2 | 8 | NULL |
+-------------+----+---------------+