When you use a sugar ribbon at the base of cake tiers, you have the choice of using fondant, gumpaste or chocolate clay. I personally like chocolate clay because it's so malleable. Of course, it's better used in cooler weather, not in the summer when it will melt.
Sugar ribbons aren't difficult, all it takes is a ruler and a pizza cutter or a knife to make an even one. If you use chocolate clay you can roll it out, cut it,then let it cool off and stiffen up a little so that it won't stretch while it's being put on the cake.
To seam the corners on a square cake, or the join on a round one, use a dogbone tool (I think that's what it's called) to smooth the edges together. If the chocolate is very cold you might have to hold the ends between your fingers for a minute to warm it up, then you'll be able to rub the chocolate so that it melds together.
The end result will be a continuous ribbon without a seam. It's easier to do this to chocolate clay than it is to do in fondant, but if you use some crisco and work carefully at it, you can usually smooth out fondant the same way. It's generally a more noticeable join, though.