If you have a sugarcraft gun, then use the rope attachment to make the rope border.
If you do not, then roll out 3 narrow strips with the flat of your hands and twist the pieces together. Don't worry if it doesn't go all the way around. You can do it in pieces and use a little water to 'glue' it together - it won't be noticeable.
Then 'glue' the strips on the cake with a little water. Do a little strip of rope for the seam at the back. And you will also do this for the top of the cake when the time comes.