FWIW...
Properly done, metal lining a holster can lengthen the life of the holster by reducing the contact the gun makes with the inside of the holster.
How?
Metal linings have one main purpose, to hold the pouch open to facilitate a faster draw. Whether or not this is good is beyond the scope of this post.
The pouch by virtue of the metal lining is kept away from the cylinder of the gun which allows the cylinder to spin as the hammer is cocked at the start of the draw.
The metal lining is not there to retain the gun.
The metal lining is usually only big enough to cover the cylinder area of the gun.
This has advantages in the old style fast draw competitions a-la Andy Anderson/Arvo Ojala, but has little practical use in CAS type shooting (we move a lot and the gun needs to be secure in the holster) (Although I've done it by request)
Steel liners are SASS legal. If you have doubts call the Sass Office and ask for Hipshot.
Steel liners are probably not allowed in NCOWS or other authenticity minded organizations.
To make one,
Your pattern for the metal lining needs to be just the right size so that when the lining is stitched to the face of the holster, it is just inside the stitch line by 1/8" to 3/16".
The pattern should have a rounded profile so there are no corners.
If you want a flared out pouch lip, use a ball peen hammer to sculpt the lip of the metal liner (before gluing to leather)
use a dremel tool or something similar to round all the edges to prevent the lining from cutting into the leather.
glue to the lining then glue this to the face.
You now have a flat holster pattern with a metal liner.
CAREFULLY, mold the holster around your form until the welt is formed so you can stitch it.
continue holster as you normally would.
I use 24 gauge sheet metal.
It's a real pain and adds to the cost of the holster.
In my opinion it does not offer enough to outweigh the trouble to make one.
The above is totally reflective of my experience and teaching, but not necessarily the last word on metal linings.