Three are some example XSLTs with your install. Take a look in the "<<your language here>> Extras/Examples/XML Examples/Import" folder. These may or may not work for your source xml, but they may give you an idea of what the XSLT are like for IMPORT. The "T" in xslt means to transform (from one xml schema into another xml schema, text or html. The import is mostly a "mapping" of xml elements with the FM fields. If there are repeated elements in your source, these are handled by "loops" in the XSLT and each loop would import a record.
You can find the DTD for FMPXMLRESULT grammar in the ../Examples/XML Documentation folder. Don't worry about the FMPDSORESULT grammar as it is deprecated and cannot be used IMPORT.
Without knowing your database (tables and fields), a sample xslt for your particular needs cannot be posted here. Try the examples first.