[JerryG]

How do you go about achieving this? The only way I can get the shaft inline with the right forearm is to either have the right hand in a "strong" position or to have no part of the right thumb pad overlapping the left thumb.

I would suggest setting the right hand (assuming you are right handed) on the putter so your forearm is in line with the shaft and take a few practice putts just with your right hand with the number one and three pressure points directly behind the shaft. Then add the left hand to the shaft with the shaft running up through the lifeline of the left hand. Both thumbs should rest on the top of the grip. It may help to initially run both forefingers down the length of the shaft.
I wish I could explain this better. Dary, O.B., Kev, City, can you help?