Download the JDEV 12c installable file from Oracle
Please Note :- Below installation screen shots shows how to install Jdev. This will not support SOA extension like we have in 11g.
You will be prompt to below screen
Select a Oracle Home directory
As installation move, you will see the progress bar
Summery page will show you option to start Jdev from here itself or you can select later/customization options.
Reference oracle documentation – http://docs.oracle.com/middleware/1213/jdev/install/install.htm
Starting Jdev 12C java, J2EE applications.