How should i quilt a string quilt?

Wow, that's gorgeous. Hers is not quilted in straight lines. If yours has to be, due to equipment limitations, I would suggest going straight along the edges of what appears to be a square or diamond. It's not made of blocks (or it doesn't look like it is) but the eye will divide it that way when you look at the whole thing. Stand back, look at your quilt, and see where your eye runs. Quilt along the lines your eye uses to divide the whole into manageable sections.