How to create a wsdl file

The simplest option is to navigate to the web service url in the browser and add a querystring parameter wsdl to it.
example: http://localhost:1831/Service1.asmx?wsdl
And the wsdl will be emitted on the browser.
The other way is to use the disco.exe utility.
example: disco /o:C:\outputdir http://localhost:1831/Service1.asmx
to run disco open the visual studio command prompt.
the syntax is disco.exe options urlToDiscover
/o or /out specifies the output directory to save the discovered documents in. The default is the current directory.