Ranga Babu has a few methods for starting SQL Server in single-user mode:
It is advisable to use SQLCMD when you want to query SQL Server that is started in single user mode as connecting directly and query using SQL Server Management Studio that uses more than one connection. To query SQL Server single user mode using SQL Server Management Studio, open SQL Server Management Studio, and do not connect to SQL Server directly. Close the connection window and click on New Query as shown in the below image which opens a query editor in SQL Server Management Studio:
I recommend practicing this a few times, as the only time you’d actually start SQL Server in single-user mode is during an emergency and that means people breathing down your neck (figuratively if not literally).