I agree with juve11, his method will keep the gap from closing up , When you look at the the fit it will seem to large, just put the shield down and weld , it wont seem so wide while your welding .
Using a decent gap, 5/32" is the way to go, but I don't agree with bridge tacks. I don't use them. Tack up in 4 places, feather your tacks, examine the gap critically and put the tightest quarter(of 4) where you would want to weld this one 1st. Some say it is easier to go from 6 o'clock to 9 o'clock or 6 o'clock to 3 o'clock, whatever, i usually weld the seemingly more difficult quarter first, this is where I would put the tightest quarter.
Run hot-125 amps and be deliberate, not hurried. Weld the bottom quarter, then the other bottom quarter and then either of the top two quarters until the root is in. To me, you need to minimize the ocillation of the tungsten concentrating the heat right on the rod as it is melting. I firmly believe in "walking-the-cup", but even if you have not much experience it shouldn't matter as long as you don't run the arc up the side of the bevels. Crank up your heat 20-25 amps and put in your hot pass.
You can usually tell if you are getting in good by watching the puddle carefully, it should be swirling and ofcourse , always use a sharp tungsten.
My 2 cents.
Be well,
Hawk