I have explained scrum methodology in brief. Do check it out. A team must share good communication amongst each member. The product owner explains his needs in a story.
The team plans to release the required user story or a sprint expected by its owner. The two main techniques used are the Fibonacci series and T-shirt sizing. In the Fibonacci series, the next number is the sum of the first two numbers, for example, 1,2,3,5,8,13,21 and so on.
The complexity increases with increase in difficulty at each level. These two techniques are used commonly. Other techniques are also available.