How do I tell the app how long my supplier takes to ship the products (Lead Time) and how many days of inventory I want to keep on hand (Days of stock)

You can save these values in App Settings to update the app regarding how long your supplier takes to ship the products (Lead Time) and how many days of inventory you want to keep on hand (Days of stock).

The default 'Lead Time' is 14 days and 'Days of Stock' is 30 days, which is common for all products.

You can update the default from Settings > Forecast Settings:

If you want to update the 'Lead Time' & 'Days of Stock' separately for any individual product/variant, you can do this from forecast results page, as shown in GIF below:

You can also select multiple products or select all products from page to bulk edit the 'Lead Time' and 'Days of Stock' values.