Caring for second/third year grafted trees

Last spring I cleft grafted some little apple scions onto little rootstock - due to critter munching the trees each got to about a foot (or a little more) high. I want to eventually train them into open center trees (I know most apple trees are modified central leader, but for some reason everyone in my part of Northern California does and teaches open center pruning for pome fruits).

I realized today that I have no idea how to prune, or even if to prune, the one-year old little trees.  I know how to prune mature trees or those three and four year old, three foot high trees from a nursery.  But what to do with these little ones their second year? Do I just let them grow, or make heading cuts, or what? They’re about as thick as my pinky finger with the rootstock being a little thicker.  Any advice or direction to resources is much appreciated.  I guess this is something usually just dealt with professionally in nurseries.

I never prune a tree, grafted or not until they are at least 4 years old.
In California they open center prune for two reasons, 1. ease of harvest, 2. more sunlight gets to every fruit.

If you don't know how to prune, you need to get a book and study up, butchering a tree because of ignorance of proper techniques is a crime. (not punishable except that many have killed their trees by creating problems by improper pruning)
Your local extension service should have a good pamphlet on pruning trees available for free or just a few dollars.

Redhawk