How to use the 13-week forecast
Put today's bank balance in the starting cash cell and the week-starting date at the top. For each week, enter the cash you expect to collect and the cash you expect to pay. Totals, closing cash and next week's opening cash are formulas.
Update it every week: replace last week's forecast with what actually happened and add a new week at the end, so you always look 13 weeks ahead.