There are a number of steps to creating a new publisher package, and these have articles of their own.
Go to your Institution page and click "+ New Package."
Select what kind of publisher you want to add (currently Elsevier, Wiley, Springer Nature, SAGE, and Taylor & Francis are supported; all publishers will be supported by spring 2021), and give your publisher a name. Click "OK" and wait for the new publisher to be created.
Click "View" on your new publisher, then select the "Setup" tab.
Upload the COUNTER report for that publisher (there's a separate article on how to do this, with way more details).
Select the Forecasts tab, and click "New Scenario." Then make sure all the parameters are set to your liking. In particular, you'll want to change the Big Deal Cost and Growth numbers from the defaults, which you can do by clicking Parameters➞Cost➞Big Deal Cost and Parameters➞Cost➞Big Deal Growth.
That's all that is required! You're ready to explore the impact of cancellation on your costs and fulfillment.
We also recommend that you
load any custom à la carte prices that your institution has been offered (especially if you are a larger institution, where custom prices can be much higher than list price). There's a separate article about that.
load journal-level perpetual access dates (especially if you are a smaller institution, where these can differ a lot from defaults). There's a separate article about that.