In order to use the Cash Flow Forecast Template from P&L and BS, follow these steps:
Be sure to connect the company you want to see data for in the Profit & Loss tab and Balance Sheet tab (learn how to do that in this article)
Add your opening cash balance in cell B15 (red arrow below)
Select your client's accounts in the drop down menu (boxed in red below)
Add your expected income (Cash Inflow) and expenses (Cash outflow) for each month
Your ending cash balance will automatically update
Ending Cash Balance =
Opening Cash Balance + Cash In - Cash out)