This article will show you how to set that up: 

Setting up primary, client-level and electronic filing passwords in ProSeries 

You'll need to setup the Primary password for yourself first (think of it as an Admin password that typically the firm owner sets up). Then client-level passwords can be setup for any returns you want to limit access and visibility to. Most of the data for password-protected clients shows as **** on the HomeBase (except for Client Name)