Be sure to mark your tacks on the pipe so the radiographer knows where they are. With gringers not allowed for the test, it's likely "some" allowance will be given.
When leaving a tack, as starting at 6 o'clock, it's pretty straightforward, waiting until the end of the tack, about a half-puddle's worth, is molten before adding filler. Arriving at three and 9 o'clock is only a bit trickier... Hold off on the filler leaving the last keyhole, and heat at the end of the tack until molten, then move downhand as you feed the last keyhole. Arriving at 12 o'clock is easier... It's the same as 3 and 6, but with gravity on your side, suck-back is not a concern.
For x-ray, even under ASME IX, you mostly need to be sure to break the edges and fill the root flush to the inside of the pipe. Some internal reinforcement is preferred, of course, and the WPS will tell you if it's required,but I've even seen a slight suck-back pass when the cap reinforcement was adequate. Again, be sure to check the WPS, and if possible ask the proctor about acceptible standards. I was once looked out on a weld I know would have passed bend and x-ray by an extremely picky examiner.
Hope this is some help. Do you have opportunity to practice?
Let me dig a bit... It seems to me Jody has a recent video (root only) where he does it without feathered tacks, using a big 'ol air-cooled torch on his scratch-start rig with the home-brew rectifier.
Edit-
Okay, it's actually on carbon steel, but may help with the technique...
http://www.youtube.com/watch?feature=pl ... YYkPELEBRU
Steve S