Add a dollop of buttercream on top. Use your icing spatula to spread it out into a very thin layer on top of the cake, pushing excess buttercream over the edges. Spread the buttercream very thinly around the sides of the cake, adding more buttercream as needed, until all bare spots are covered.
Take your bench scraper, hold it at a 45-degree angle to the side of the cake, and sweep it around the side as you spin the turntable. Scrape any excess crumb-filled frosting off the bench scraper and into a small bowl (not back into your bowl of clean buttercream).
If there are gaps, or uneven areas, add some buttercream to that spot (using what you scraped off), and smooth it out again with the bench scraper. Use your bench scraper or icing spatula to smooth the top of the cake, too.
Your cake is now covered in a crumb coat. It should be very thin, so that you can see the cake through the buttercream. Spin the cake around, and check that it's level and standing straight up on all sides. If it seems to be leaning to one side, use your bench scraper to gently push it straight, then smooth the sides again.
Refrigerate the cake for at least 30 minutes, until the crumb coat is chilled, and no longer feels sticky. If it's a warm day, and your buttercream was warm and soft to begin with, you may need to chill the crumb coat for 45-60 minutes.
Cover the bowl of remaining buttercream with plastic wrap so it doesn't crust over. If your kitchen is warm (hotter than 72 degrees Fahrenheit), place it in the refrigerator to keep it cool.
While you're waiting, make yourself a fresh cup of coffee, and decide how you want to decorate your cake.