For most arcs, say 7 to 15 DME, fly 10 degree increments, the mantra is "twist 10, turn 10." After making your initial turn onto the arc, fly a straight line segment that is actually tangential to the arc. As your OBS needle swings from full scale to center (or, from plus 5 to minus 5), you turn another 10 degrees heading, and twist the OBS selection 10 degrees. You have to think about which way you're going, and which way to turn the OBS dial. Kind of hard to explain without pictures, hope this makes sense.