I found a Celtic knot in a coloring book.
I copied and enlarged it to fit a sheet of paper. There are two cords in this knot. I made five copies and cut out the outlines of the cords.
I glued the lighter outlines to the darker outlines. For one outline, I put a spot of glue at the spot where the outline is under the other one. For the other outline, I cut open the spaces where the outline is under the other one.
I used the fifth copy as a placement guide. I positioned one outline on the guide, then put the other on top of it, gluing the two outlines together.
Slide the cut ends under the other outline, and the knot is complete.