Download the latest set of JavaPOS Device Controls from this site.
Get the appropriate JavaPOS supported devices you need.
For starters, check the device list identified under "Resources and Device Support" (but remember that many additional JavaPOS supported devices are not yet listed). All these devices have JavaPOS Device Services available from the manufactorer.
Read the initial chapters of the latest UnifiedPOS specification, and the Java mapping of this specification (JavaPOS) which is contained in Appendix B.
Read those sections of the UnifiedPOS specification that correspond to the device models you will need.
Examine the structure of the JavaPOS application contained in POStest.
Write your JavaPOS-compliant Point of Sale application, using the defined Java APIs, and linking in the JCL and the supplied Device Controls. The JCL config file should specify the vendor-supplied Device Services that match the actual POS devices your application will be using.