Quote - How complicated is it to do this? What are the steps?

It's actually very easy to do.  I was surprised by the simplicity.

From Appendix 5 of the Antonia~WM documention:

Antonia Polygon – Standard and Antonia Polygon – WM share geometry with Antonia 1.2. Conforming clothing for those characters can be converted without geometry changes using this simple process:

 

• Load the geometry of the clothing item as a prop.
• Open the Set-up Room.
• From the library change the figure to Antonia Polygon – WM, (or use the development version). To the question whether you want to re-group answer no because the groups are the same.
• You now have a weight-mapped conforming item. Re-name it and test it (without conforming) in the pose room. If it behaves the way you expect it to, save it to your figures library.
• Exit and re-start Poser.
• Test the piece with it conformed to an Antonia Figure.
• If you want the piece for further use, open the Set-up Room and trim the rig down to the bones you need.
• You can transfer any morphs from the existing clothing in a separate application like MorphManager.