Need help for creation user decision by using class

Hi Friends,

They is 1 custom workflow need to develop for z module pool screen.So i previously i used to develop z business object for this,but this time for learning purpose i want to use classes.

So i have created z class and write code on that,every thing working i use user decision step for Approve or Reject.

Right now when i execute the work item it automatically opening z screen first after that workflow screen(Approve or Reject).

I need only workflow screen.when i click link in attachment in work item that time only i need to open this Z screen.

Please suggest any solution for this.

Thank,

Naresh.K