I've done a bit of it. Is the forecasting flat (eg take annual figure and spread 1/12 to each month) or are there increments (eg correct for CPI every 3 months)?
Generally, you need a form where you set up the items, total value, amount to spread (could be less than total is you already have actual data), start month, end month.
Then loop through that table, spreading across the interval from start month to end month and writing values out to a forecast table.
By having categories for the items you can then create a grouped report showing detail and summary by category, by month.