As you plant, add more rocks. Since this is a rock garden, the look I'm after is a mass of rocks with plants popping out between the cracks. But if all the rocks were laid at once, they'd be in the way of the plantings.
So instead, additional rocks (mainly small ones) are inserted as the planting proceeds. Install a plant somewhere first, then create the "cracks" around it afterwards. It's a lot easier to change your mind and move a rock to another position than it is to dig up plants and replant them somewhere else.
There's a delicate balance to pursue between the planting and the rock installation, but your goal ultimately is to cover as much of the surface as possible with rocks and plants. The process is rather like putting together a jigsaw puzzle -- except that you determine how the puzzle turns out.
Once you've covered the entire surface of the rock garden with plants and rocks, it's time to mulch, the subject of Step 9....