Upload it the same way you upload projections, then on Ultimate build a field from it and point your Contest Sims at that field.
Before the steps, what uploading ownership does on its own: it changes Adjusted Ownership and, through it, OLD Saber Score. That's all. It does not change Saber Score, which runs against an internal field that can't be edited, and it does not change Sim ROI until you do steps 2 and 3.
1. Upload the ownership. Click the upload projections icon, upload a CSV or paste your data with a column named Ownership, and name the source. Uploading again under the same name overwrites it. Leave Apply to All Slates off, since ownership changes with every slate.
2. Build a field from it. Either in the same upload window, turn on Generate Field from Custom Ownership, or click the “gear” icon in the “My Own” column header and click “Generate Field From Ownership”. SaberSim builds a set of field lineups, which are simulated opponent lineups, from your numbers instead of ours.
3. Point your Contest Sims at that field. Open the contest's sim settings and change Field Lineups to your custom field. Contest Sims use field lineups rather than ownership numbers, so this is the step that makes your ownership show up in Sim ROI.
Related: How do I upload my own projections? · What happens when I change ownership? · What is the difference between Adj Own and My Own?