Armor for a horse's head.
This image shows part of a pattern I saw on a chanfron at the Cleveland Museum of Art. It's not exactly knotwork, but it is similar in some ways.
A chanfron is a piece of armor that goes over the face or head of a horse. The one this pattern came from belonged to Sir Henry Lee (1530-1610). It was made in about 1585 by Jacob Halder out of etched and gilded steel. The background was a silvery-gray steel color, and the pattern was etched and filled in with gold. It was pretty cool, and the picture you see here doesn't begin to do it justice.
In fact, this drawing isn't even what I'd consider complete. But I wanted to post it, because it's an example of how to construct interlaced patterns (including knotwork).
If you look at the drawing from the bottom up, you'll see the stages I took it through.
I produced the drawing by making a quick sketch of the basic pattern, along with some notes about the relative size of each part. I also made some accurate sketches of a few details. Later, when I had more time, I used my sketches and some graph paper to get a more accurate representation of the pattern. Then, I widened and interlaced the lines, erased stray marks, and so forth. (Some of the setting-out lines are still visible near the bottom of the drawing.) At that point, I scanned the image.
To complete the image, I would strengthen the lines and, because this is a repeating pattern, I would snip out the repeating section and copy it until it was the length I wanted. Then, I would color the pattern and the background. These final steps could be done electronically or by hand, depending on the target medium.