Hey, sorry but that’s not how I do the upside down method – it’s not precise enough for me. I actually use a technique that is a mixture of two other techniques but it’s not something I’ve ever seen anyone else do.
Basically, I put ganache on the top of my cake like I normally would and roughly smooth it. I then take a cake circle the same size as the cake circle underneath my cake (I use silver cake boards) – I sprinkle it with a little water and then put it on top of the ganache and line it up with the cake circle underneath my cake, checking in several positions to make sure it’s lined up. I then flip the cake upside down. I use a spirit level to check how level my cake is and push the cake down wherever is needed in order to make sure it is level. I then take my scraper and using, what is now the bottom board (now that it’s been flipped over) as a guide, I take my bench scraper and scrape away any ganache that is overhanging this board. I then pop it in the fridge for 30 minutes or so to harden. I then take it back out of the fridge and ganache the cake as I normally would, but I now have a board on the top and bottom, so that my scraper is completely supported and I am in no danger of having sloping sides. Once the cake is ganached, I pop it back in the fridge for several hours until it is completely solid. I take it back out of the fridge and flip it back over onto the bottom again. I then carefully remove the board on top, which should be easy as the ganache is solid. You will probably find that there are little holes and gaps to fill in the top, but just take a small amount of ganache and fill these areas – don’t use too much or your’ll lose that lovely flat and level top and you’ll have to turn it upside down again – you really only want to just fill the gaps and that is all – don’t start blobbing a load of ganache on top! I then put the cake back in the fridge again and leave it until I am ready to cover it.
I hope that it explains it a bit better. The process is very similar for buttercream however instead of sprinkling the cake board with water, I actually attach (using removable adhesive on the board) a parchment cake circle that is just slightly smaller than the board and then put that on top of the cake.