As a first step, an organization needs a high-level logical data model to represent the target enterprise data architecture. Using that model, you can assign each subject area / entity to an organizational target unit that is typically responsible to create/update that entity as their respective owner. (I use the term "target" because the current enterprise data architecture - if even documented - may be siloed and also that the current organizational structure may not be optimized for future purpose.)
Each owning organizational unit should be represented by one or several Data Stewards.
Applying the above principle, the subject areas / domains of entities that are assigned to a certain organizational unit can be easily drawn, as the vast majority of entities has (more precisely: in the future should have) only one natural owner which typically is not only the creator of the data but usually also its main consumer.
However, particular attention is to be paid to the Master Data domains, especially Party. The objects of Party occur in many different roles (customer, supplier, employee etc.) and therefore have either none or many potential "owners" (Sales, Customer Service, Purchase, Human Resources etc.).
I recommend to create a separate central unit that takes ownership e.g. for all matters related to Party and represents the interest of the organization as a whole and not only of one department (the latter being one of the reasons for a siloed structure in the first place). This central unit has "the license" to define the entities/attributes related to Party, the business rules, the data governance measures etc. Other organizational units ("licensees") embed this "one and only" way of creating/updating objects of Party into their processes and supporting applications. Certainly, the Data Steward(s) representing the licensor need to consult the data stewards representing the licensees to make sure that all the requirements of the latter units are taken into consideration.
This been said, there is no reason to "boil the ocean", i.e. no need to have a complete coverage of the whole organization, before any fruitful work can be started.
I agree with a previous comment to focus in priority on areas with problems, i.e. where engaging in Data Stewardship can be justified by the ROI and/or where the COI (cost of inaction) indicates ongoing loss of money or the risk of fines (for industries that need to comply with requirements of regulatory authorities). Data Stewards for prioritized areas can start their work as a project task force and may later evolve into a formal, permanent role.