To estimate Product Backlog items Scrum does not require a specific technique.
The objective of estimation
Estimations should allow discussion to obtain more information about the product backlog items. Estimations help to create a shared agreement about the solutions. In fact, it is important that the team is commitment on the work that has been decided. Acting this way helps to reinforce team relationship and collaboration.
The most used planning technique
Many agile teams are using some form of planning poker for their agile estimation. Planning poker is an agile technique that allows to estimate and plan product backlog items.
A session starts with the Product Owner who reads a user story or describes a feature to the estimators. Each estimator has a deck of planning poker cards. Each value on the card represents the number of story points, working days, or other units used to make estimation. Then, the estimators ask questions to the Product Owner to know more about each story. After discussing the story, each estimator selects one card to represent the estimate.
In case not all estimators select the same value, the estimators need to share the reasons why they have chosen those values. Afterwards, each estimator selects a new value. The process goes on till agreement is achieved. It can also happens that estimators decide that a specific item cannot be estimated because they need to gather further information. In this case the item is postponed.
Keep in mind
Planning poker is a technique where team members estimate each Product Backlog item by using planning poker cards. If there are only marginal differences among estimates they discuss a bit to reach an agreement. Whereas, if there are outliers a longer discussion would be needed. The team repeats the process until they reach complete consensus.