Once User Stories and Acceptance Criteria are created, developers being estimating tasks. This is done in a "Planning Poker". Each team member having a set of "cards", consisting of numbers in the Fibonacci sequence, and assigning a number in this sequence to each task.
The Planning Poker aims at providing estimates as accurate as possible for each task. The common scale for estimates is the Fibonnaci sequence. Each developer having a set of cards, with numbers. Each task is discussed, and a card is pulled by each team member. If all members agree on the estimate then the games moves on to the next task. If members do not agree, then discussion is resumed and the game starts again, for the same task.
While Scrum focuses on estimating points, most other Agile methods focus on time. Bigger tasks are split into smaller tasks.
To calibrate the scale, a task in the middle (of complexity) is chosen followed by one for each end of the scale; with points agreed on them first.