Okay, first off, I am not an instructor so I may be shot to bits about this, but this is how I do it.
First fly 90 degrees to the radial that you first intercept in the direction of the arc. Set the ODB for 10 degrees further on, wait until the needle centres then fly 90 degrees from that radial and so on and so on. It isn't a true arc, more like a polygon, but it works.