If the architecture design is in place so that should answer you as what would be the next step. You need to get into object oriented design and creation. I assume you already have the framework architecture along with hardware and software requirements. Now you have to look into each tier and decide (according to your requirements) as what would be more suitable for each tier. i.e applets / HTML for client tier or think of servlets / JSP for web tier and so on.

What layer are you more interested in? if it is business tier, then you need to think of EJBs and JDOs and consider performance and security patterns.

I guess it's more your call now.