Here are the basic steps to map one type of document to another assuming you are starting from scratch. You can also go through the cheat sheets (tutorials) by going to Help > Cheat Sheets and selecting Data Mapper.
Import or Create the structure definitions by right-clicking in the Repository navigator and choosing New > Structure. Possible sources of structures are:
XML Schema (XSD)/DTD/XML Sample/WSDL - Import any of these to process XML documents. See XML Representation for more details.
CSV Sample - Import a CSV sample document. See Flat Representation for more details.
Manual - You can manually create a structure by adding elements in the structure editor. Typically used for flat (delimited/positional) data. See Flat Representation for more details.
COBOL - Import a COBOL copybook. See COBOL Considerations for more details.
Database - Import table definitions directly from a database. See Database for more details.
Java - Import Java classes or Jar files. See Java Representation for more details.
EDI - Import EDI Guideline XML (gXML) files. See EDI Representation for more details.
Projects - Import a previously exported project. Use the Import menu item for this.
Set up the map by right-clicking New > Map in the Repository navigator which executes the map creation wizard. Once the map is created, drag the structure corresponding to the input document to the input panel of the map's editor, and drag the structure corresponding to the output document to the output panel of the map's editor.
Map elements from the input panel to the output panel by dragging an input element to an output element. This causes the input element value to be copied to the output element value.
Note that after you have dropped the input element to the output element, the output element is selected and the Output Value panel contains Concat followed by the name of the input element. This indicates the output element is created by copying the specified input element.
Test your transformation by choosing Test > Test Run specifying a test document. The output is displayed on the screen
There are also extensive Examples.