If you look carefully at the plans you will see a line that runs from the top of the mainmast, down to the rail, aft of the three shrouds. This is the topmast backstay.
I’m using the next smaller size rigging line for this – as a general rule, the higher up you go on the model, the smaller the rigging line gets, at least for the standing rigging. Below is a scan of the rigging diagram. I marked the line with a red arrow.
The green arrow shows the attachment point of the stay to the rail. I believe the kit would just have you simply drill a hole here and run the line through it and glue it into place. That’s perfectly fine, but I have been adding eyebolts to the model, so that’s what I’m doing here.
The rail was drilled through, the eyebolt inserted and glued into place. The excess stick through the bottom of the rail was trimmed off. All set to add the backstay itself.
I didn’t take any photos of the backstay at this point, as it’s such a simple operation. I just took a single piece of thin rigging line, made an overhand knot in the center, passing the loop over the top of the mast, then tightened the knot. The stays, port and starboard, were then tied off to their respective eyebolts.
When doing this, you want the stay to be taught, but not so much that you cause the mast to lean one way or the other. This may take a little practice. Fortunately, the stays attaching the topmast to the bowsprit will keep you from bending the topmast backwards. It’s mostly the left and right lean you want to watch out for.