Usually you work from some existing object.
Usually this object is part of a prp file.
For example, you can build an Age in Blender and export it to a prp.
Then you can extract an object from the prp file using a libHPlasma utility such as PrpShop, prpmod, etc.
You extract it in xml format that you can edit with any editor.
To make a new object, you copy an existing xml file and then you modify it.
Then you can compile it into an uof, then import it into a prp using Prpshop.
Or you can skip the uof and import directly from the xml file with prpmod.
"The weak can never forgive. Forgiveness is an attribute of the strong."
-- Mahatma Gandhi