Apple Trees

Hi, i am trying to grow apple trees for the first time. I have planted them and would like to train them to grow in a cordon formation. I have attached a photograph of the trees to help me explain.

I have managed to get the bottom branches fixed to my bottom line but how do i get the other branches trained in order for them to grow along the middle and top lines? Do i cut off the remaining branches leaving the middle one and try to encourage the buds underneath the middle and top lines to become branches and then train them? Also what is the best time to prune these trees.

Help!!!



Regards

Mark