The bid form asks for the base bid first. Alternates, unit prices and allowances appear after it. Owners usually compare bidders on the base bid, sometimes with accepted alternates.
Example: the base bid covers the main gym equipment. A second gym is Alternate 2. The equipment bidder keeps the second gym out of the base number.
Before you price, sort every item into base bid or alternate. Read Division 01 and the bid form to see where the line falls. Addenda often move work from one side to the other.