The Pro Rata method annualizes a partial period "in proportion to" the elapsed time. For example, if ten months of the year have elapsed, the Pro Rata method increases the amounts by a factor of 1.2 (12 / 10 = 1.2).
To apply the Pro Rata method, all you have to do is set the Annualization Rate Calculation Method assumption on the Financial Statements Assumptions Sheet to Pro Rata. The annualized amounts are calculated automatically and an annualization column appears on the worksheet for the must recent income statement.