The quantity takeoff is the first half of an estimate. Pricing is the second half. Each line has an item, a quantity and a unit of measure.
A drywall estimator measures linear feet of each wall type. A plumbing estimator counts fixtures from the fixture schedule. A paving estimator measures square feet and converts to tons.
Work one sheet at a time. Mark each item as you count it. Check the schedules against the plans, because they often disagree.
Note the sheet for each quantity. That record lets another person check your work.