Generally I use sqlcmd from the command line to do it and I use a batch script to start the instance in single usermode and then immediately connect to it. I do this because if an application connects faster than you, you have to shutdown and try again. However, you can connect with SSMS as long as the Object Explorer isn't connected to the server. You do this by clicking cancel on the first connect dialog when SSMS opens. Object Explorer is its own connection and each query window is a connection, so you have to disconnect the object explorer connections to get a query window connection to the single user mode instance.