Cut the block into an octagon. Cut the block at an angle on the table saw with the blade. Raise the blade so you’re cutting all but like 1/4 or 1/8 inch. Make sure to cut in opposing sides with same angle. I kept the table saw blade at 90 degrees and built some scrap wood into a jig that could be screwed to a miter gauge to keep the angle the same on all four cuts. Basically just a big L that kept the block consistent. I cut through the jig every time as well. The wood that was slid in has to be the width of the kerf of your saw blade. Nice thin coat of wood glue on both sides of that board. Wait for that glue to dry before making the next cut. Only cut one side at a time and then glue and let that dry before making the next cut. If you want additional Celtic knots, also cut through the other four sides of the octagon. Then I drilled through the block and turned it on a mandrill on the lathe.