1. Create the login
First create a SQL Server login for your domain account.
CREATE LOGIN [your domain account] FROM WINDOWS;
GO
your domain account has to be replaced with the domain account you want to add. For instance your domain is called darth and the user vader than it would be CREATE LOGIN [darth\vader] FROM WINDOWS; GO;
To check if the login was successfully created type in
SELECT NAME FROM SYS.SERVER_PRINCIPALS
GO
and the domain user should appear in the result list.
2. Grant sysadmin rights
You can retrieve a list of server roles using the sp_helpsrvrole procedure. So to add the user to the sysadmin server role group type the following (example is still domain darth user vader)
SP_ADDSRVROLEMEMBER 'darth\vader', 'sysadmin'
GO
Done.
Quit SQLCMD and open a non elevated instance with your domain account. Type in select user_name() and the result should be dbo.