methawee I am not such that there is such a general guide on this. There are a lot of different cases, and specificities of each database. I think that you have to prepare that yourself, and figure out which data from which schema go to which table/columns in your new merged schema. Only you can do that, because only you knows your models.
And doing so, ask yourself these questions:
- How do I deal with duplicated ids and primary keys?
- How do I deal with constraints?