I freehand with a large bread knife. Here is how I do it.
Take the bread knife and score all around the cake where you want the cut to happen, keep turning the cake following the score mark letting the knife slowly work its way through the cake without pushing the knife too much, it will eventually make its way all the way through.
Another way is to make the initial score mark and then with a long piece of dental floss or thin string, put it in the score mark, wrap around the cake, and pull the ends of the string in opposite directions.
The score mark helps with the string especially if your cake has a thick outer crust.