C#
<%@ WebService Language="c#" Class="MyServiceNs.WebServiceSimple" %> 
using System.Web.Services;

namespace MyServiceNs
{
  [WebService(Namespace="http://localhost/myservice")]
  public class WebServiceSimple : WebService 
  {
    [WebMethod(Description="This method sums 2 integers")]
    public int Add(int a, int b)
    {
      return a+b;
    }
  }
}

- create WebServiceSimpleEmbedded.asmx file and copy paste the code sample above
- create c:\intepub\wwwroot\WebServiceSimple folder and copy the .asmx file under the folder
- Open IIS Manager
- expand Default web site node
- right click on WebServiceSimple and select Convert to Application. This will associate an Application pool with your web service.
Option 2: split the .asmx file above into header in .asmx and code App_Code\*.asmx.cs
Option 3: compile web service code into a .dll a put it under \bin folder

To see web service definition in SOAP, type http://localhost/WebServiceSimple/WebServiceSimpleEmbedded.asmx?WSDL

XML
<configuration>
  <system.web>
    <webServices>
      <protocols>
        <add name="HttpGet" />
      </protocols>
    </webServices>
  </system.web>
</configuration>

HTML
<FORM action="http://localhost/WebserviceSimple/WebServiceSimpleEmbedded.asmx/Add" method="post"> 
  A <input type="text" name="a"> +
  B <input type="text" name="b">
    <input type="submit" value="Calculate">
</FORM>

In order to call web service from a .net client you need to create a proxy. You can do it from command line

CMD
"c:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Bin\NETFX 4.0 Tools\WSDL.exe" 
   http://localhost/WebserviceSimple/WebServiceSimpleEmbedded.asmx?WSDL 
   /out:WebServiceSimpleProxy.cs 

another option is to create web service proxy from Visual Studio by right clicking on the References folder and selecting Add service reference

You need to include WebServiceSimpleProxy.cs that defines "WebServiceSimple" client class in your project and compile the code below

C#
using System;

namespace Client
{
    class Program
    {
        static void Main(string[] args)
        {
            WebServiceSimple client = new WebServiceSimple();  // class defined in proxy 
            int result = client.Add(1, 2);
            client.Close();
        }
    }
}

C#
public class SampleService : System.Web.Services.WebService
{
    [WebMethod(Description = "This method sums 2 integers", EnableSession=true)]
    public int Add(int a, int b)
    {
        if (Session["AccessCounter"] == null) { Session["AccessCounter"] = 1; }   // using Session requires "EnableSession=true" attribute
        else { Session["AccessCounter"] = ((Int32)Session["AccessCounter"]) + 1; }

        return a + b;
    }

    [WebMethod(EnableSession=true)]
    public int AccessCount()
    {
        if (Session["AccessCounter"] == null) { return 0; }
        else { return (Int32)Session["AccessCounter"]; }
    }
}