When you install SQL Server, the installation creates an instance (DEFAULT OR SQLEXPRESS) of SQL Server. In my case I have created instanace named DEVSQL.
Some times we do need to create multiple instances based on requirements. To create more instances, start the setup again. The following screen appears when setup starts for the SQL Server 2012.
Click “New SQL Server stand-alone installation or add features to an existing installation”, the following screen appears:
As you can see DEVSQL instance already exists, it was created by the first installation, now select “New installation or add shared features”. Then click Next and accept the License Agreement.
Then select features you want to install for the new instance.
After that the installation offer you to specify the name of your new instance (MYTEST was chosen in this example):
Click Next to create the new instance and perform the remaining setup steps.
Now you are done!