how to add ASHX file (custom request handler) to sharepoint 2013 and create URL Mapping

Hi everyone. I have a question regarding ASHX files to SharePoint 2013.

I followed few examples online (made for SP2010 version). Are there any specifics for 2013 version?

Because when i try to open ASHX file (url link for example something like ... http://<myportal>/_layouts/15/<myProject>/myHandler.ashx)  i get a runtime error.

Thank you in advance.