There are various ways to do this.

1. Find the PID for process that is hosting the service. You can do this with the help of command,
   tasklist /svc
   
   Look for service and associated PID, then go to command prompt, into the directory where windbg is installed and give command,
   windbg.exe -p <ProcessID>
   
2. Another way is to directly use the service name as parameter to windbg. This is more convenient way, but hardly known to users/developers. You can use following command,
   
   windbg.exe -psn RPCSS
   
   Above command will find out the process which is hosting RPCSS process and then attach the debugger to that process.

Stay tuned..