In its simplest form, the steps to a release plan are:
Write enough stories to define a successful product.
Do any necessary exploration.
Estimate the difficulty of implementing each story.
Estimate the speed of story implementation.
Choose stories for the first release based on business value and difficulty.
The purpose of the release plan is to prime XP's continuous planning circle of life. No plan can predict the future perfectly, but planning can help you steer the project to success. Think of it this way:
If you knew how long it would take to build each desired feature in your product, you could do a really good job of picking the best mix of features given your time and budget.
No one can tell you exactly, far in ...