Look at the "SOAP query example" or the WSDL from the SharePoint web-service (e.g. .../_vti_bin/lists.asmx?op=GetListItems
or ../_vti_bin/lists.asmx?op=GetListItems&WSDL
) and then look at the corresponding MSDN documentation (such as GetListItems) on how to "use" the web-service.
It takes a little bit of familiarity to "know" to map viewFields
with <viewFields>...</viewFields>
(most work like this), but... the MSDN documentation (if prodded carefully) says "what" to put in the XML where the WSDL just gives the near-useless outline. There are a number of examples in the tubes (and related SO questions) of hand-rolled SOAP access for SP for various tasks.
Microsoft also has some Open Specifications -- the link is always hard for me to find. Lots of stuff under the SharePoint branch. YMMV and it's mostly white-paper, but a good resource.
Not sure what tools Flex has but because of the limited WSDL support, most of the mapping has to be hand-coded or come from a better definition source -- hopefully "an existing library" which can be used directly or modified-to-suit.
I would highly recommend using a tool for testing the service access -- e.g. soapUI, which actually has a horrid UI -- because even the littlest error will come back with a cryptic error messages. Also, make sure to use SOAP 1.2.
Happy (less than maximal pain) coding.
P.S. A more specific question about a specific web-service would likely yield better responses.