Create WEB-INF\flex folder to your ASP.NET application.

Add services-config.xml to the WEB-INF\flex folder
If you are using Flex Builder the project should be created with "Flex Data Services" option enabled, the flex project "Root folder" set to you asp.net application path and the "Root URL" set to the Url of the asp.net application.

Specify the additional Flex compiler arguments:
-service "Path to your asp.net application folder\WEB-INF\flex\services-config.xml"
(The path must point to the services-config.xml file located in the WEB-INF\flex folder of the ASP.NET application)
-context-root "/your asp.net application name"

The services-config.xml is included in the source code distribution:

<?xml version="1.0" encoding="UTF-8"?>
<services-config>
    <services>
        <service id="remoting-service"
                 class="flex.messaging.services.RemotingService"
                 messageTypes="flex.messaging.messages.RemotingMessage">
            <destination id="fluorine">
                <channels>
                    <channel ref="my-amf"/>
                </channels>
                <properties>
                    <source> *</source>
                </properties>
            </destination>
        </service>
    </services>

<security>
   <login-command class="com.TheSilentGroup.Fluorine.Messaging.Security.GenericLoginCommand" server="asp.net"/>
</security>

<channels>
        <channel-definition id="my-amf" class="mx.messaging.channels.AMFChannel">
            <endpoint uri="http://{server.name}:{server.port}/{context.root}/Gateway.aspx" class="flex.messaging.endpoints.AMFEndpoint"/>
        </channel-definition>
    </channels>
</services-config>