Learn how to draw celtic knots:

From Wikipedia:

"Celtic knots are a variety of (mostly endless) knots and stylized graphical representations of knots used for decoration, adopted by the ancient Celts. Though Celtic knots were being created in Polytheistic Celtic times, these knots are most known for their adaptation for use in the ornamentation of Christian monuments and manuscripts like the 8th century Book of Kells."

Here are some tutorials from various sources:

Felt tip method:
No erasing. Draw solid colored lines, then outline to create the knot.




Double strand method:
Draw lines with staggered gaps, then connect to create the knot.




Dots method:
Use dots and arcs to create a celtic border or column.




Graph method for square knots:
Use graph paper, lines and arcs to create celtic knots.




Graph method for round knots:
Use graph paper, dots and arcs to create celtic knots.

And finally, some reference photos: