Stay with me on this Black River, . . . not complicated, . . .
Open up your holster pattern so the inside is face down.
Lay the pattern down on a piece of 8 oz leather so that just a tad bit is peeking out along the sewing edge of the holster you are making for your revolver.
This is the outside sewing edge, . . . not the one next to your leg.
Outline the edge of the holster edge from top to bottom. Cut that line.
Next take a pair of dividers and lay out a 3/8 inch wide piece using the cut edge as the outside edge.
Cut that line, . . . lay it on the inside of your pattern, . . . you have a welt.
Now, . . . there are times that it is a little wider, . . . sometimes narrower (not much), . . . sometimes thinner, . . . sometimes thicker, . . . but this is a good "starting" place.
Cement it in, . . . sand the edge smooth, . . . edge / burnish / sew, . . . voila, . . . western holster with a welt.
This is the basic procedure for my western rigs.
May God bless,
Dwight