Stone Veneer preperation???

Help!!! I am going to start redoing my fireplace, but I was hoping to get some tips first. Currently, I have a rock/boulder (80's look) fireplace that goes up to my ceiling. What I would like to do is install the stacked stone veneer over the existing rock, but I am not sure what prep work needs to be done first. I plan to do it in two basic steps..., First put mortar on the existing rock and make it a flat surface until it hardens. Then install the veneer on the new flat mortar wall. I am just worried that it won't stick. Do I need to use some type of concrete bonding agent between layers? And what else am I forgetting. Any help would be appreciated. Thanks.