Scrum does not prescribe specific estimation techniques. 

Estimation by analogy is one technique used by Scrum teams.  The mechanism is very simple and consists of comparing backlog items to similar backlog items that were previously implemented.  Lack of available data points will limit the effectiveness of this technique. 

Planning Poker is a popular iterative approach to estimating.  Here are the steps:

• Each estimator is given a deck of cards, each card has a valid estimate written on it
• The Product owner reads a story and it’s discussed briefly
• Each estimator selects a card representing his or her estimate
• The cards are turned over so all can see them
• Discuss differences (especially outliers)
• Re-estimate until estimates converge

Planning poker cards and an online version of Planning Poker are available from Mountain Goat Software