How to add second receptacle to the same line?

I know it sounds rudimentary, but I'd like to make sure I'm doing it right.
I have two dedicated power lines with one duplex receptacle on each, that I want to add another two duplex receptacles to.
I have an idea, but not sure it's the right one. I don't really want to call an electrician for that. I installed receptacles before, so that shouldn't be a problem.
thanks in advance