I generally only make fondant covered cakes but I have been getting a lot of requests for frosting only cakes lately and I am looking for some tips if anyone has any. In particular, what would be your best advice for making a cake like this:

If I were icing that cake, I would basically ice that cake smooth (and level) and then put in the curved vertical design with a small spatula.  Might take you a bit to get the hang of it, but heck, that's the beauty of buttercream or SMBC.  You can smooth it out again and start over if you don't like the looks of it.