I’m preparing myself for migrating old Odoo 8 instance to a new version 12. It should be done in the span of couple next months so there is some time for the preparation. 

I looked up into OpenUpgrade project from OCA:
https://github.com/OCA/OpenUpgrade

But to be honest the documentation over there is really poor and do not explain almost anything (or am I missing something?).
I’m aware that this is difficult process and should be performed by knowledgeable staff but still - the “documentation” does not document anything. And I have the time needed to get to know the procedure and test everything, so it is not like I want to get new version up and running tomorrow.
And from what I know there is some custom stuff in that old instance.

I saw some comments like the one in this topic:
https://www.odoo.com/forum/help-1/question/how-to-do-odoo-database-migration-by-using-https-github-com-oca-openupgrade-122800

But I would really appreciate an actual example. For example on how fill the parameters of the migration script etc.

Is there maybe some manuals or other sources I could read/watch to learn about this stuff? I know this is very broad question but I’m now aware of any other place I could get the knowledge I need.

Thanks