The first thing I do with my cakes is level each layer after it has cooled to room temperature.
An optional step is to stack each layer and trim the sides (after leveling the tops). I don’t do this with all cakes, but when I do I start by slicing a thin vertical strip down the side. Then I hold the knife vertically and slowly turn the turntable cutting away the outer crust of the cake starting at the spot where I cut away the vertical strip. This helps keep each layer the same diameter when finished. Doing this will also help get that 90 degree edge you’re looking for. A lot of people say trimming the sides like this is not good to do, but if you’re careful the cake turns out just fine. If you go slow and occasionally cut the side strips off as they get too long, then everything should be fine. It may not be a perfect circle after this, but if you keep the knife vertical, and the edge of it just inside the crust ring around the edge as you’re trimming, then it will be pretty close to a perfect circle. Besides, after you frost the cake nobody will know it wasn’t a perfect circle. I’ve made many cakes and skipped this step and they still turned out just fine.
Once I have the cakes leveled and the sides trimmed (if I trimmed the sides) I apply a thin coat of frosting to the cake to create a crumb coat. For the layer that will be the bottom layer of the cake I put a thin layer of frosting on the cardboard the cake will sit on and that helps hold the layer on the cardboard until there’s enough weight from the other layers. I just make sure not to go all the way to the edge of the cardboard so that it doesn’t have a messy edge to it.
To help keep the cardboard clean while I decorate the cake, I cut wax paper (you can use aluminum foil, or parchment paper if you want to) into strips about 3-4 inches wide and 6-8 inches long and then carefully slip them between the bottom layer of cake and the cardboard. It’s not too hard to do if the strips aren’t all folded and crinkly. Just hold the strips by their edges at the long ends keeping the strip tight enough to stay straight. Don’t pull too hard, though, you’ll end up tearing your strip. Wax paper works really well because it’s already slippery, but the advantage of using aluminum foil strips is that you can fold the edges around the edge of the cardboard until you’re ready to remove them. It should take 3-4 of these strips to go around the whole cake if you’re using a 10” round board. Looking at the cake from above the strips will look like a triangle or square and there ends of the strips will overlap each other a little bit. If you’re making a rectangular or square cake this step is even easier to do.
For each additional layer I place them on a sheet of wax paper and apply a thin crumb coat just like I did with the first layer. Once the crumb coat has been applied I put the frosted layers in the freezer for 5-10 minutes until the frosting has stiffened. Once the frosting has set then I start frosting each layer the rest of the way. The advantage of doing this is it makes frosting the cake really easy without getting a bunch of crumbs in your filling layers. The disadvantage to this is that it adds more time to the overall process. In my case, my freezer usually only has room in it to hold one layer, maybe two if I don’t have a lot of food already in there. So what I do is I start with the bottom layer and put it in the freezer as soon as the crumb coat has been applied. Then I start applying the crumb coat to the next layer and put it in the refrigerator. If there’s more than two layers, I’ll start the next layer. After that I take the first layer out of the freezer for frosting and move the other layer from the refrigerator to the freezer, and the third layer into the refrigerator. Then I apply enough frosting for my filling layer to the bottom layer. By then the next layer is ready to come out of the freezer and the other layer can be transferred to the freezer. The wax paper peals easily from the cake layer right out of the freezer.
Keep repeating this until all of your layers are stacked. After I have all of my layers stacked I apply a crumb coat to the side of the cake and put it back in the freezer for 5-10 minutes. Once this is done then I put a ton of frosting on the top in the center and use an offset spatula to spread the frosting. As the frosting reaches the sides and falls over the edge, I spread that around the sides for the final frosting layer. As long as you’re not pushing too much of the frosting over the side at one time it will stick to the side of the cake and it will be easy to spread around the side of the cake, especially if you are using a turntable of some sort.
Now you’re ready to put on any final touches with a cake comb, nuts, or whatever you like to do for decorating and garnishing purposes. Once I have everything like I want it I put it in the freezer one more time for 5 minutes to stiffen the frosting again. After this last trip to the freezer my strips are ready to be removed from underneath the cake. Gently pull at first, once you feel it starting to slip from underneath then keep pulling and you’re in the clear. And then, voil?, you have a finished cake ready for serving. If it’s going to be several hours or longer until the cake is going to be served, I put it in the refrigerator, especially if it’s a warm day and I used a buttercream frosting. Otherwise I’ll let it sit out at room temperature before it’s served.
This is the way I like to do things, but others here have their own style, too. I think everyone here has provided a lot of good advice to you. I suggest you try the tips you like and see what works best for you. After you do a few cakes you’ll figure out which tips worked well for you and which ones didn’t. Then you’ll find your own style and frosting a cake will be second nature to you.
Good luck and have fun with your cakes, silke!