Draw an equilateral triangle. An equilateral triangle has sides that are all the same length and each point has a 60-degree angle. Draw the triangle lightly so that it is easy to erase later. To make an equilateral triangle, draw a straight line and measure a 60-degree angle at either end of the line using the protractor. Make a mark at the 60-degree angle and draw a line from the end of the original line and through your mark. If done correctly, all sides of the triangle will be the same length.
Draw a curved line from one point of the triangle to the next. The curve must be deep enough that the curves overlap and form a rough triangle in the middle. You can draw the curves by hand or use a compass. If you use a compass, measure to the middle of one side of the equilateral triangle, then draw a perpendicular line that goes outward. Place the point of your compass at the end of this line and draw the curve. Repeat and use the same distance for each side for consistency.
Erase all of your guide lines, leaving the knot pattern in the middle.
Turn the paper so that one "loop" in the trefoil knot is parallel. At the bottom of the loop, erase a small section on either side of the line intersecting it. Repeat for each loop of the knot. If you have done this correctly, it will look as though you have a continuous line that forms all of the loops; the erased portions give the visual impression that the line is moving over or under other lines.
Embellish the trefoil as desired. Celtic trefoils are often elaborate and decorative. You can use a line parallel to the original to make the image look more three-dimensional, add decorative curves or make another knot inside of the original. Any method of decoration is acceptable.