The two needed tools are a MockUp service that can be built from the WSDL of the remote web service and an HTTP Sniffer. While you can find similar tools written in a variety of languages the two that I am presenting hereare written in Java and thanks to that they run in any major OS out there by this time.
No matter what you use (DotNet client, RoR client, PHP Client, Spring-WS, AXIS2, AXIS, etc) the steps below should help you.
Writting use cases is important so you know every detail as to how your application should work. They are also a perfect initial documentation that even if outdated in the future at least gives other developers comming after you or even to yourself a reminder as to what the application does.
Building the Mockup Services (necessary to simulate Use Cases)
1. Start from understanding the services and ending points we will use from the remote service. In other words visit the WSDLs (for example https://domain/some/path/descriptor?wsdl)
2. Use SOAPUI to create a new project giving the WSDLs (File|New Project|Provide WSDL and a name|Check "Create Requests" and "Create MockService").
3. Locate from the Request group the remote method to invoke and double click on "Request 1". Complete the request parameters if any and click on the "Run" button
4. Right click on the root of the resulting entry on the right panel and click on "Generate MockService". Locate from the MockService group the remote method to invoke and double click on "Response1"
5. Copy the response text from the right pane of the request window into the right pane of the response window.
6. Repeat the above for any single method you want to Mock up (simulate)
7. Double click on the MockService group (or right click and select "Show MockService Editor"). Hit the "Run" button and the service will be available on port 8088
8. At this point from the Request window the address can be changed to point to the local Mock Service listening on port 8088. Hit the green button called "Submit request ...". The right pane should show up whatever the Response window right pane contains
This is a java utility which comes with Axis (not Axis2). To run it use something like:
java -cp $CLASSPATH org.apache.axis.utils.tcpmon 8081 localhost 8088 &
With the commands above you get a window where you can see the TCP traffic going back and forth to port 8081. That port is proxing port 8088 meaning that requests to made to 8081 are actually forwarded to 8088.
Just go back to SOAPUI and click on any "Request 1" window address bar, pick the local address of the mockup service. Click again on the address bar and select "edit current", change the port number to 8081. After hitting the 'Run' button you can see the response on SOAPUI but you can also get the whole request and response including HTTP headers from tcpmon. Of course this is not that useful because you have all that information from SOAPUI already but when you start building your client you will not longer see your requests on SOAPUI.