A really quick way to make your cake look finished is to hide the seams. The seam is here, you can see where it's 2 cakes put together, but we want it to look like one giant piece. I'm going to use a rose tip here, which is usually number 104. I've got some royal icing in my piping bag. You'll notice that it's pushed all the way forward and then spun around, and that's to keep it from shooting out the bag and covering my hand and anything else that might be in the area. Nice clean tip. You're going to take the fat end and just go towards the end. I'm filling it in. I know it looks a little crazy at first. I'll now take my finger and just drag it around, getting rid of any excess. It's also going to make it nice and sturdy. There we go.
I've done the entire top tier. I'm just going to go around and take off any excess. You want to go fairly quickly that it doesn't bubble up too much on you. Royal icing dries up like cement. It's what we use to build gingerbread houses, so it's going to hold everything together really nicely, but you want to fairly quickly go around and clean up the edges. Also, continually clean off your tip, it's going to keep any errant Royal icing from getting all over the place. You'll notice I'm going fast, but not too fast. I want it to really fill in the seams.
There we go, nice clean finger. If I go too fast, then it will look a little bit dry, so just keep going back, almost like you were caulking a bathtub. Just fill it in. You can use some of the excess Royal icing to take care of any elephant skin that you might have gotten. It'll fill in the little holes or any little pin pricks you did to get rid of bubbles. Just brush it in, and it will cover up a host of problems.
There you go. That is how you hide the seams between the tiers of your cake.