There's several previous threads on this site that cover the basics of this, but judging by your photo, the first thing you need to do is widen that seam a lot more. The usual advice is a 12:1 taper from the old to new work, so you have sufficient bond surface to marry the two. This means if the 'glass is 1/4" thick, you'll need 3" of taper on each side of the line, more if it's thicker. An angle grinder does a good job of this, particularly if fitted with a diamond blade. Once tapered in both directions, you'll apply enough fabric to bulk it back up to just below flush of the surface. Typically you'd like to see a combo fabric (1208, 1708, etc.) employed to get the most strength across the seam. Lastly the very shallow depression is filled with fairing compound and smoothed to satisfaction.