Here I'm putting in some 1/4" holes near the top of my posts. These holes will be used for some stainless steel rods that will act like a fence to keep things from rolling away.
 

Each post will have two holes that need to be 90 degrees apart. What you see below is one method of doing this. I placed a pin in the first hole, made sure is was parallel to the table and then put my second hole in. Note my work stop at the end. This helps locate all my holes exactly. Also note the plastic jaws in my vise. These plastic jaws have a V groove in them to locate round parts and they don't harm delicate work pieces either.
 

My daughter and I talked about how she was going to fasten this shelf to the wall and we decided to put the screws under the shelf so they don't show. Then it hit me while I was getting ready to drill the screw holes, what if she miss' the location by a small amount?

What I came up with is to use slots in place of holes (arrows). This will allow a small amount of movement and should provide an easier install for her.
 

It's Finished

Here it is hanging on her wall. And as you can see, she added some cool looking stencils to the maple area.
 

You can also see the shelf is loaded up with stuff as well. The only thing left for her to do is put some clear on it and then it should last for many years. This was another fun project that was a collaborate effort between the both of us.