how to sort year and month order by in sql

Please refer below code.

SQL

DECLARE @test AS TABLE(Id INT,billdate DATETIME)
 
INSERT INTO @test VALUES(1,'2015-02-02')
INSERT INTO @test VALUES(2,'2015-03-07')
INSERT INTO @test VALUES(3,'2015-05-04')
INSERT INTO @test VALUES(4,'2015-06-08')
INSERT INTO @test VALUES(5,'2015-08-08')
INSERT INTO @test VALUES(6,'2015-09-02')
INSERT INTO @test VALUES(7,'2016-05-02')
INSERT INTO @test VALUES(8,'2016-04-05')

SELECT CONVERT(VARCHAR,YEAR(billdate))+'_'+ LOWER(DATENAME(MM,billdate)) as [Date] FROM @test
ORDER BY YEAR(billdate),MONTH(billdate)

Output

Date

2015_february

2015_march

2015_may

2015_june

2015_august

2015_september

2016_april

2016_may