An iteration planning meeting is called at the beginning of each iteration to produce that iteration's plan of programming tasks. Each iteration is 1 to 3 weeks long. User stories are chosen for this iteration by the customer from the release plan in order of the most valuable to the customer first. Failed acceptance tests to be fixed are also selected. The customer selects user stories with estimates that total up to the project velocity from the last iteration.
The user stories and failed tests are broken down into the programming tasks that will support them. Tasks are written down on index cards like user stories. While user stories are in the customer's language, tasks are in the developer's language. Duplicate tasks can be removed. These task cards will be the detailed plan for the iteration.
Developers sign up to do the tasks and then estimate how long their own tasks will take to complete. It is important for the developer who accepts a task to also be the one who estimates how long it will take to finish. People are not interchangeable and the person who is going to do the task must estimate how long it will take.
Each task should be estimated as 1, 2,  or

3 (add 1/2 if you need to) ideal programming days in duration. Ideal programming days are how long it would take you to complete the task if there were no distractions. Tasks which are shorter than 1 day can be grouped together. Tasks which are longer than 3 days should be broken down farther.
Now the project velocity is used again to determine if the iteration is over booked or not. Total up the time estimates in ideal programming days of the tasks, this must not exceed the project velocity from the previous iteration. If the iteration has too much then the customer must choose user stories to be put off until a later iteration (snow plowing).
If the iteration has too little then another story can be accepted. The velocity in task days (iteration planning) overrides the velocity in story weeks (release planning) as it is more accurate.
It is often alarming to see user stories being snow plowed. Don't panic. Remember the importance of unit testing and refactoring. A debt in either of these areas will slow you down. Avoid adding any functionality before it is scheduled. Just add what you need for today. Adding anything extra will slow you down.