Celtic knots are a lot of fun to decorate in grayscale or colour.
In these first three examples, I used repeating patterns in pen & ink.
Other than pen & ink, you can use graphite for shading.
Here, I divided the strands in half, and shaded one side.
In this one, I shaded to enhance the illusion of overlap of the strands by having darker shading where one strand goes under another.
And in this one, by shading darker on the edges and lighter in the middle, I can give the illusion that the strands are dimensional tubes.
In part 2 next Monday, I will show you some ideas using colour.