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An XML schema defines what elements go into an XML file and in what structure; in this case, it may be helpful to think of it as a document which lists all the fields that go into a database record. For instance, it might stipulate the top-level element is <member>, which contains <name>, <zipcode>, and <lastpurchase>, and <lastpurchase> contains <date> and <amount>.
Since XForms has limited browser support, you'll need a script running on your server which will accept form input and generate properly structured XML from it. The input form is the same as any other input form-- just provide fields for the data to be entered. The work of formatting the XML is all in the script-- no transformation is required.
On the xsl-list archives, see [biglist.com...]
As for where to get such a script, it's outside my skillset, but it's a fairly trivial task.