To forecast in your Planning grid, you'll just need to change the view to Actuals + Forecasts - you can show Actuals To any month you like, or none at all. If you show any actuals, then you will not be able to forecast under these months.
In this example, Actuals are set to August, which allows the months from September to June to have forecasts entered. Nothing can be entered in July and August as those are the months where actual information is pulling in from Xero.
To forecast in your production/livestock trackers, you'll need to head to your Operations > Trackers > Select your tracker in question. From there, you'll have a menu to Add Forecast Transaction, you can then enter stock movements as you see fit.
In this example, the actuals are set to June 2021, a month prior to the July season start. This allows for the full-year forecast screen to be viewed as indicated by the orange header.
If you want to quickly build out a forecast in your tracker (rather than the transaction screen shown above), toggle the Monthly view in the header section to enter those transactions for the desired months.