The normal way of upside down frosting was to torte/fill/crumb coat, go freeze for a few hours, frost and then freeze again. Now I Torte/Fill/Crumb coat/Frost and freeze with only one trip to the freezer instead of two! Hopefully this will help make it clear. Unfortunately I can't save you time looking at all these photos...sorry, don't do videos :-))).
First I start by preparing my cake board. Then place a piece of tape in the center and cover with baking paper.
Spread enough frosting to surround my 6inch cake and from here is where I torte/build my layers on what will eventually be the top of my cake.
Once stacked I level my cake with a sharp knife all around, using the top layer as my guide to prevent cake from showing once I begin my upside down frosting. Next, my cake board goes on what will be the bottom layer and I just kind of eyeball it all around as I turn the table to be sure it's centered. The width of the board will be the thickness of your icing all around so if you don't want a lot of frosting you can trim the circle down a bit.
At this point I use my leveler to be sure I don't have a lopsided cake once frosted, placing one hand on the turntable and the other lightly (because your cake isn't frozen) on top to keep my leveler steady and centered as I turn. Just make sure the bubble is balanced evenly between the two black lines as you rotate your turntable, the bubble will only leave those two lines (moving left or right) if your cake isn't level. You then gently press down on your cake to get the bubble centered. Doesn't need to be perfect, just make sure for the most part the bubble stays between those lines.
Starting from the bottom (which will be your top) use your offset spatula to push icing in and up around your cake. Then overfill all around to get a nice even coat of icing when you do the next step.
Now start taking off that extra icing using the top (which will be the bottom) as your guide. Placing one hand on top gently and turning. I found since the cake wasn't frozen I had to take a smaller portion off at a time.
Once the cake has been frozen, I use a hot spatula (dipped in water), pressing in/upward to gently remove the board from the cake, then I flip it over.
Any creases on top are taken care of with a dollop of frosting and spread with a hot spatula (normally it turns out smoother than this). Smooth any bubbly sides the same way.
Once the sides are done you'll have frosting at the top edges. A little tip from Lauren Kitchens is to come up from the side and swipe across the top, but each time you do be sure to completely wipe off your spatula (a clean spatula is the key), I always place mine in warm water and then wipe.
And there you go!