LT, do you have an M3 or M7 holster? If is an M3 (which is WWII/Korean War era but perfectly usable for Nam) then its a simple affair in which you place the only strap diagonally across you chest with the holster under/around the wearers left armpit.
However if its an M7 holster (Vietnam era onwards) then place the holster under the left armpit, put the shorter strap (which may have a 'leather pad') over the left shoulder and the longer strap around your chest and clip in on the front of the holster. Here is an old photo of me wearing an M7...
Hope it helps mate. I'll see if I can find any other photos that are clearer.
I have to admit, I started out with an M7 (which was supposed to be an improvement over the old M3) and hated it. The strap over the left shoulder kept slipping and the chest strap dug in at the back. I ended up tracking down a black M3 holster to replace it and boy and I glad I did! Love mine now!
VIETNAM: Ia Drang Part 2: Broken Arrow - The Long Walk - Choker - Ho Chi Minh - Overdrive - Battle of the Chuối Gà River - Operation Diamond Head
COLD WAR: Cutthroat - Black Gold Part 2: Two Faces - Hockwerk - Double Barrel Part 1 - Operation Light House - Battle of the Weser - Gulag - DMZ 84 - The Dark
Non Gunman Events:
VIETNAM: DorkingNAM Nov. 2009 - DorkingNAM April 2010 - CS 'In Country 8' Aug. 2010 - CS 'In Country 9' Aug. 2011 - DorkingNAM Oct. 2011 - DorkingNAM Nov. 2012 - Operation: Dust Off April 2014 - Operation: Dust Off II May 2015
OTHER: CS Operation "Full Metal Wedding Jacket" June 2012 - WWII The Battle of Tulagi April 2016 - Grenada Day 2 August 2016