This is a fairly open-ended question. 

You will need to decide on your user interface--it could be ASP.NET, InfoPath, or somethine else.  In your first approval client event, your user interface will need to allow the user to choose the next approver.  Once the next approver has been chosen, you will need to assign this value to a datafield in K2.  How you do this depends on what technology you use for your UI.  For the next approval step, use the variable where you saved the  approver in your destination rule. 

There are several places you can go for more information:

1.  The tutorials included in the blackpearl help

2.  The Developer's Reference

3.  Sample projects .