help to write UDF function

HI Experts,

I am new in sap pi, do not kow how to write UDF.

In my complex mapping scenario, I need to write UDF.

Source IDOC :

Z1WHCONS

   Z1WHCUSTC

     Z1WHCUSTCH

       Z1WHCUTM

           comment

       Z1WHCUTM

           comment

       Z1CUSTCD   

Target:

BOX                  --     Z1WHCUSTC when Z1WHCONS is initial

MIXPLT            --      Z1WHCONS

    BOX               --  --     Z1WHCUSTC when Z1WHCONS is not initial

       Line              --        Z1CUSTCD

         comment   ---    Z1WHCUTM-comment field

         comment   ---    Z1WHCUTM-comment field

I am not getting proper data when BOX and BOS with MIxplt data is coming in source xml.

How can I do it using graphical mapping or UDF.