Relative estimates  This suits well for iterative software development projects. Relative estimates with planning poker is most famous way among this kind of estimation.
Here is the following steps to do relative estimates though planning poker.
 We need to give some points to user story/functional points based on the complexity. The points are based on relative numbers by comparing the user stories complexity based on our previous experience.
These relative points are arrived from input from team by using planner poker technique. This is similar to Delphi technique, here individual member is asked to give relative points with out knowing what other member is quoting. Then the numbers will be shown to all and based on that discussion, once again team will be asked to give points again. This process can be repeated till we arrive at an agreement.
We can use planning poker cards to arrive the numbers. These numbers are similar to Fibonacci serious 0,1/2,1,2,3,5,8,13,20,40,100. The idea of these points to have some division of numbers from zero to 100 with consistence intervals between such that we can give good relative points with sufficient variation.
After points are arrived a consolidated estimation can be arrived based on what is the estimation for single point and number of iteration can be arrived based on this rough estimation.
And before each iteration each team will be taking different user story based on product managers ordering. A detail estimation of user story will be asked from the team, since factors like team is different, there might some unknown the new teams might come across needs to considered.
Now it comes a concept called velocity, velocity means if team A choices user story with total counts of 20 points, then velocity of the team is 20. velocity might vary based on the team experience and knowledge level and situation. Ideally the velocity will be consistence after the team completed few iteration.
With out using cards we can use sites like http://planningpoker.com/ to get planning poker estimates.
Here is the following steps to do relative estimates though planning poker.
 We need to give some points to user story/functional points based on the complexity. The points are based on relative numbers by comparing the user stories complexity based on our previous experience.
These relative points are arrived from input from team by using planner poker technique. This is similar to Delphi technique, here individual member is asked to give relative points with out knowing what other member is quoting. Then the numbers will be shown to all and based on that discussion, once again team will be asked to give points again. This process can be repeated till we arrive at an agreement.
We can use planning poker cards to arrive the numbers. These numbers are similar to Fibonacci serious 0,1/2,1,2,3,5,8,13,20,40,100. The idea of these points to have some division of numbers from zero to 100 with consistence intervals between such that we can give good relative points with sufficient variation.
User
Stories from product Back log

Relative
Points

User Story A

5

User Story B

13

User Story C

20

User Story D

8

User Story E

40

User Story F

2

User Story G

100

User Story H

5










After points are arrived a consolidated estimation can be arrived based on what is the estimation for single point and number of iteration can be arrived based on this rough estimation.
And before each iteration each team will be taking different user story based on product managers ordering. A detail estimation of user story will be asked from the team, since factors like team is different, there might some unknown the new teams might come across needs to considered.
Now it comes a concept called velocity, velocity means if team A choices user story with total counts of 20 points, then velocity of the team is 20. velocity might vary based on the team experience and knowledge level and situation. Ideally the velocity will be consistence after the team completed few iteration.
With out using cards we can use sites like http://planningpoker.com/ to get planning poker estimates.