How to allocate the fee from one class to another 2 classes? finally, how to do a related journal entry to allocate in QBO?

ex, there are 3 kinds of classes: overhead, design, service. I chose the class when doing income and  expense entries. Since overhead expense are related to design and service classes, at the end of month, I want to allocate overhead expense to each of design, service according to the total sales.  how can I do that in QBO? and how to record a journal entry to allocated the overhead expenses to Design and Service. 

Simply, I have to calculate and distribute overhead fee into design and service abiding by the total sales, then record a journal entry to allocate the fees calculated and distributed to classes: design and service. How to realize this?  thank you so much!