[Martytoof]

I don't think you're going to find a step by step tutorial, but once you get comfortable with a sewing awl the process should be a fairly simple stitch job.

Find some buckles with a leather base, break a small section of stitching that your current buckles are being held in place by, place the new buckles into the pad where the old buckles were, stitch the pad closed with a fair amount of overlap between the old stitching and new stitching so the break in stitching doesn't begin to run.

Repeat for the other side of the pad, but sew in the strap, or another buckle so you can buckle the strap on both sides, or however you want to accomplish this.

I wouldn't recommend breaking any existing stitching on an expensive goalie pad until you've become comfortable with awl stitching though.