1 - Define the metric, Enter metric and amount data types.
2 - Create a new rollup field.
3 - Specify the rollup field to track against goals.
4 - Specify details, about the source data that rolls up.
5 - Specify the date field that determines the goal period that the records witll rollup into.
Reference:
https://docs.microsoft.com/en-us/dynamics365/customer-engagement/sales-enterprise/create-edit-goal-metric