How can we create a Custom Report Template in SSRS?

A report template also exists as a normal report (RDL file). If you want to customize your own report templates, you can put the previously prepared reports (which will be used as new templates) in a specific folder. 

By default, the path is:

C:\Program Files\Microsoft Visual Studio 8\Common7\IDE\PrivateAssemblies\ProjectItems\ReportProject 

Then, to use these templates, you can right click on the Reports folder in your project and select New Item. You will then see the name of the report templates you added in the above folder.