To make the Trinity knot we are going to use a compass. The compass makes the knot perfectly symmetrical and easy to create. This type of knot is most commonly called the Trinity knot because of the Holy Trinity (Father, Son and Holy Ghost). But it also may refer to gods of even older origin: the old triple gods and goddess' of the ancient Celtic religion. Regardless of its name, you will find the Trinity knot very common in old manuscripts. It was used everywhere, as part of bigger knots, and as free standing knots. In each panel I will show you step by step how to create a Trinty knot. Each new step will be in red. As you go through the panels, old steps will turn gold, and there will be a new step highlighted, again in red.
     
To make the Trinity knot, you will need a compass and a piece of regular paper. Be sure to protect your working table underneath from the point of the compass, or the compass will dig little holes! Make a fair sized circle on the page, say at least two inches in diameter. Make sure there is a bit of room around the circle yet on your page, as the resulting knot when we are done will be slightly larger than this initial circle. Do not adjust your compass after you have drawn this circle, as we will need it to be the same size for the next circles we draw.
Mark the circle at approximately the twelve o'clock position. Place the point of the compass on this point and use it to make marks where it crosses our initial circle on each side. You do not have to draw the whole circle, just where it would cross our first circle.
Now place your compass on one of the marks you have just made, it doesn't matter which one you do first. Draw a semi-circle within our initial circle. It should start at the twelve o'clock point and end in the lower quarter of the circle. You don't have to continue the arc outside of the initial circle, you only need to draw what falls within our starting circle.
Make the other arc the same as you did the first one. The two arcs should cross at the centre point of your circle. If they don't, check to make sure you have not changed your compass setting...it should still be the same size as our first circle.
Placing your compass point on the lower tailing end of one of your arcs, mark off another tic on the bottom on the circle. You don't need to do a mark on either side of the arc tail, just on the lower half. If you want to verify your dimensions, you can put your compass on the other arcs tail and check to see that it would make a mark in the same place on the bottom of the circle, but you shouldn't have to. So long as you've kept the compass size the same it should all line up.
Place your compass on this bottom mark and draw another arc from side to side within the circle. This is the basic skeleton of the Trinity knot, and all we have left to do is double up the lines and erase our overs and unders.
Enlarge your compass diameter now, by however much you'd want the thickness of the strands of your knot to be. Place the compass point back onto the marks we made in the upper half of the circle. From each point, draw another arc within our circle, and extending a bit beyond. It is important now to make sure that you extend the arcs a bit outside of the circle, so they will meet up when the arcs are all drawn. Draw these new arcs from both of the upper marks, and from the mark on the bottom as well.
Pick a point where one of your knot strips intersects another, and make it pass over the other, erasing the under lines from the "under" strip from within your "over" strip. The next pass for the knot strip, following the same strand, will be to go under the next intersection, so erase appropriately. The last pass will again be over, so erase the underneath one.
At this point the initial circle can be removed, as well as the marks for our arcs. You can place your Trinity knot within a circle by using the centre point of the old circle, and making it whatever size you want. It also fits nicely within an equilateral triangle, or you can let stand alone.

Source: http://www.aon-celtic.com