How do I use Prevayler

The Prevayler release is just one JAR file. You just need to add it to your CLASSPATH and you can start working immediately.
Create your domain model. It should not depend on any external resources specifically not Prevayler itself. It also needs to be serializable.
Create a Transaction (Command) object for each type of modification to the system. They need to be serializable.
Create a Prevayler instance and execute all your transactions through it.

For further information, read the 2-minute Tutorial.