Slots for the strings make this super easy to put together. It will sit flat because knots fit inside the parts. You can print both parts together at the same time. Support material is not needed.

You can use a variety of string lengths. See the images with string lengths that are used in the photos. The easiest way, I believe, is to make the three outer strings and then determine the length of the shorter, center one. If you make the shorter one a little shorter than it has to be, it will make the outer ones tight which is a nicer look.

Does it get any easier? I'm a frayed knot!