I talk about this all the time, but here we go again, a short video that shows how I get my bees to build straight comb off bare foundation (when I have drawn comb).
Keep in mind that this method is not necessary. Many beekeepers simply let the bees build comb out from the middle frames as they naturally expand the brood nest and build comb for honey. That works too. Inserting empty frames between drawn comb is only meant to speed up the process because of the bees’ compulsion to fill in empty space.