WebSphere Application Server v8.5 uses IBM installation manager (IIM) for installation. If you are new to Installation Manager, please go trough my old post on IIM
The installation process is not changed from the previous version (WAS v8.0), so please check the previous post of WASv8 installation,
WAS v8.5 ND trail Download
This trial has all features enabled. Download the trial for an evaluation period of 60 days.