I said I'd show how to do a celtic knot but when I did this one I learned what doesn't work. The pictures still give an idea how it all works but this one didn't work very well.
The first picture shows how to cut the wood. The angle can be anything from 20 to 30 degrees, I like a 25 degree angle best. It also shows how the insert piece fits in the cut.
Then the second pic shows how to clamp it for glueing. The third is a picture of the V blocks for clamping it. Let each glue joint dry for an hour or so before making the next cut.
You make the same cut four times turning the wood 90 degrees each time (onto each side). You're cutting through the other glue joints on the 2nd, 3rd, and last cuts. The fourth picture shows what it looks like after all 4 cuts have been made and all the glueing is done.
But what I realized on this piece is this. The piece that you glue in should be the same thickness as the saw kerf from the cut. My saw takes about a 1/8" kerf and I was glueing in pieces of 1/4" baltic birch plywood. This makes the piece grow each time you glue in another piece and the result is that the rings don't line up. It still gives an interesting design, but not the celtic knot I was trying to do. So I guess I was just lucky on the first one I did by using inserts that were pretty close to 1/8" thick.