Usually it’s done on a monthly basis i.e. you note the opening bank balance at the start of the month, plus cash coming in, less cash going out and the resulting bank balance at the end of the month. You can do it for as many months as you like and you will be able to see what should be your bank balance well into the future. Many people do the forecast on a spreadsheet or there are many apps you can use.
Once you’ve put it together, it doesn’t end there! You can work with the numbers to fix any issues, such as a negative balance, if you don’t have an overdraft in place. You can speed up payments from customers, slow down payments to suppliers, inject some of your own funds or borrow externally etc.