The goal of OOUX workshop/methodology is to map and point all the object within a project, but also all the items and interactions that are related to this object. It's a good way to control missing informations, and a good way to prepare the next steps of your project (data modelling, database, screenflow, mockup, etc.)
The "howto" is described within the Miro Board, but in short, it's a step by step listing:
Start with the main Objects of your system.
Continue with the Core Contents defining you objects.
Define the Metadata that allow you to filter or order those objects.
Define the relation with other Objects to create linking and relations.
Define what are the main Interactions with this object.
It's ok to go back and forth between the different columns and object definitions. You will forget things while working on this structure, complete later.
All the definitions are given by the sourced articles and the instruction within the board.
Well… it depends. Usually you can easily cover the first aspect of a mapping in 1 to 2 hours. Oftentime you will build it and update it during the first week of mapping if your project is big.
It is a tool for which regular update and small picking is totally ok, you shouldn't spend more than 6 hours to be totally complete.
Again, it depends, but I totally suggest you to invite different kind of people from elsewhere than your own team: business analysts, designers, developers, etc.