Simple stuff actually for most ADF users but this post is for the absolute beginners.
Here is the sample Taskflow design and we need to pass parameters from Caller TF to Callee TF.
Go to the definition of Callee TF and goto Parameters tab and define the Input Parameter Definitions.
Now go to the taskflow design and click on the Callee TF and go to the properties tab. In the 'Parameters' section you will see the parameters that you defined earlier. Here you should fill in the values that you want to pass.
On the event of which the Callee TF is called, you should set the values in the pageFlowScope.param1 and param2.
<af:commandToolbarButton id="ctbCreate" action="dialog:showWizard" >
Now you would be able to get the values of param1 and param2 as 0 in the callee TF. Simple.