Set up different hourly rates in Timeslips

need to set up different hourly rates for one employee so that when a particular client is used, it uses the correct rate per hour, 

IE - Joe Blow is the employee

       A&A construction is $ 23.00 an hour

       B&B construction is $25.00 an hour

I enter a time slip for June 22 for Joe Blow working 4 hours for A&A construction and 6 hours for B&B construction.  I need the time slip to multiply the hours by the correct hourly rate.

I use one item description ( but have different pricing lists in that description)

I know it can be done, as I have done it in the past, but can't for the life of me, remember how to do it.