Keith
OK, here's my quick guide to 1937 pouches:
New webbing, as yours appears to be, is a bit stiff, but you should be able to get your finger through the tunnel....
... then if you squeeze it a bit, it'll open up. That should be enough to thread the strap through, but if not, you could use something to force it through, as suggested by others in the list.
I find the best method is to use the 'quick release' design feature of Mills webbing and just feed the brace through the upper slot on the buckle, and then down the tunnel....
... and then you can squeeze the brace strap and pop it into the buckle like this. The basic rule of all Mills Equipment Company webbing is that the end of the strap always goes through the open part of the buckle
This is what you should then have, from the back....
... and from the front.
The basic setup.
The buckle with shoulder strap hook attached.
Basic infantry configuration, with haversack, waterbottle and intrenching tool from back....
... and front.
The same set, but with the Wireless Set 22 battery carrier in place of the haversack, back view...
... and front.

Hope this is helpful to you.

Good luck with it!

Rob