I cut the square cake to the same thicknesses and followed the same process as the rounds, but was very careful at the corners. The sharp edges and corners of a square cake are really what make it look nice. I filled the cake with raspberry jam that was dammed in by a border of piped buttercream. I smoothed my crumb and final coat using my offset spatula. To smooth the sides I did not turn the cake wheel but used my bench scrapper front and back to gently pull across each side, smoothing and evening out the buttercream. It was difficult to keep the edges crisp but can be done with a lot of practice.
After my final coat of buttercream I piped a simple shell border along the top and bottom edges of my cake.