Step 1: Always start with a properly trimmed and layered cake (by cutting off the center bump from your cake). Crumb coat your cake and put it in the fridge for about 30 min or more to create a hard crust, the harder the crust the better. 

Step 2: Frost your cake with a good amount of frosting (don't worry it will mostly scrape off) on all sides. Make sure that the frosting goes above the height of the cake on all edges. 

Step 3: Put the Bar (base; see images below) of the ProFroster against your cake board as you would your bench scraper, and measure where the riser should be.

(The reason why you put it against the cake board is because the cake board is always slightly larger than your cake in diameter, this allows enough room for a good amount of frosting on the outside of the cake. By aligning the ProFroster to the edge of the cake board, the top of your cake will be the same diameter as the bottom of your cake, which is what gives you straight lines all the way down the sides of the cake. In the above video I am using an 8" cake board with an 8" cake. If you want more frosting on the outside of the cake then go up to a 9" cake board)

Step 4:  Adjust the Riser height to meet with your frosting / cake height, and allow the pins to lock into place (see images below on how to assemble the ProFroster). Don't move your hand / grip. Start rotating the cake stand but keep the ProFroster stationary and start scraping. 

Step 5: After your first pass, if you have any gaps fill them in with frosting and rotate / scrape again. You may have to do this several times to make sure that all your gaps are filled in.

Step 6: Smooth the top of your cake where the Riser has left a mark, if any, with your spatula.  

Note: Im a perfectionist. So I put the cake in the refrigerator so that it crusts again and then I frost a thin layer and scrape to make sure that all the gaps are filled, and the edges are sharp.

The cleanliness of your edge will depend on the consistency of your frosting, the thicker the frosting the better...if you use a frosting like ganache, the edge will be sharper.