How to enable -T1118 trace flag in SQL Server 2012?

SQL Server Configuration Manager can be used to set the SQL Server start-up parameters. You can specify the trace flag there:

1. Start -> Run -> SQLServerManager11.msc (or just search for SQL Server Configuration Manager)

2. In SQL Server Configuration Manager, click SQL Server Services.

3. In the right pane, right-click SQL Server (<instance_name>), and then click Properties.

4. On the Startup Parameters tab, in the Specify a startup parameter box, type the parameter (in this case the trace flag -T1118), and then click Add.

5. Click OK.

6. Restart the Database Engine.

Have a look at this link for details: http://technet.microsoft.com/en-us/library/ms345416.aspx