Explanation
To forecast at the product hierarchy level, you need to configure the forecast set on the Advanced Account Forecasting Setup page. You can select the product hierarchy field as the forecast context field, which determines the level of granularity for the forecast. You can also select the forecast metrics, such as revenue and quantity, and the forecast periods, such as monthly orquarterly. References: Salesforce Help: Configure Advanced Account Forecasting, Salesforce Help: Set Up Forecast Contexts

The first step in providing insights to the account reps is enabling metric groups. Metric groups are collections of metrics that are relevant for a specific business scenario or use case. They allow account managers to view and compare agreement terms for different metrics, such as revenue, volume, margin, and so on1. Enabling custom metrics and allowing account reps to add agreement terms are not the first steps, as they require metric groups to be enabled first23. References: 1: Create Metric Groups for Sales Agreements4, 2: Map Custom Fields of Sales Agreement Products and Schedules, 3: Select Metrics to Display in Agreement Terms.