XML Gateway - Outbound E-invoicing XML

Hello,

We are implementing an outbound e-invoicing solution (AR). We have done all the setup including creating hub, trading partners, etc. The xml is being generated, however, we want to extend/customize this xml by adding few more fields.

to do so, we have downloaded XML Gateway Message Designer, AR_INV_UBL21_OUT_001.xgm and AR_INV_UBL21_OUT_001.dtd. I have been able to do the mappings by including newer fields in xgm file in Message designer and have updated dtd as well.

I am confused about what else is required? can someone please help me with clear steps ahead? Do we need xslt mapper? How do I use it in JDeveloper? what is it exactly used for while updating xml?