Once you have done the general setup 7.2.5 Setup Forecasts (Sales/Site Goals) follow these steps:
If your fiscal year is based on the calendar year: Reference the Class "CampaignItemRevenueSplit" in Apex Class and set your schedule to what you need.
If your fiscal year is different from the calendar year but you forecast on the fiscal year period: Reference the Class "CampaignItemRevenueSplit" in Apex Class and set your schedule to what you need.