First steps: find Wikimedians to support you, create accounts, and prepare an on-wiki project page.
In many cases, there will already be some content (media files and/or data) from your collections on Wikimedia projects. Check what's already there to avoid duplication.
Wikimedia projects only host content under free licenses. The material that you contribute must comply to that. In any content partnership, good copyright clearance is key.
Select the data and media files you want to contribute, and prepare them to be compatible with Wikimedia Commons and Wikidata.
People, places and other concepts in your data need to be 'matched' with their corresponding Wikidata items.
Wikimedians can add valuable additional descriptions to your files and data. How do you invite them to do so?
Files on Wikimedia Commons and data on Wikidata can be re-used in many ways. How to encourage this?
How can you track the impact and re-use of your media and data on Wikimedia platforms?
