The most common method is to tie the roots around the rock, and add soil to the base of the trunk. Then remove the soil gradually from the top. Here is an example with an Acer buergerianum, started in March 2018 with a seedling that I didn't really know where to go with. Not very good, but it will help you understand the idea of it. I used raffia that rots away in a few months' time, but you can use other kinds of strings, and I put it in a square pot for the roots at the base of the rock to spread :
http://krizic.eu/bonsai/photos/index.php?/category/362
What I did with my cotos is a bit different : I tied the roots around the rock, and added some soil (mainly peat and sand), then wrapped the whole thing with plastic sheet.
June 2008 :
October 2008 :
I think I already posted these photos, i don't have recent ones.
And it's better to use a stone with holes and lines so that the roots can be more firmly clasped to the rock.