Set your RMI to the relevant NDB or VOR. At the turning point (usually two miles before the arc) turn until the RMI is showing the beacon as abeam. Allow the needle to drop 10 degrees behind then turn in to raise the needle 10 or 20 degrees above the abeam. Allow the needle to drop 10 degrees below abeam and repeat until reaching the lead radial. Don't forget to take wind into account.
Or as DFC says, just use the DME groundspeed. Allow the groundspeed to drop to zero. When it increases to 7, turn 10 degrees. G/s to 0 and repeat. For your average piston single or twin this will keep you to within 0.1d on the arc!