String quilting is usually done on a foundation of some sort. The foundation can be muslin (left in place ), paper (which is removed after sewing on the "strings") or even some other fabric that you don't really like, but won't show through the strings.
The easiest way is to cut a square of the foundation, lay the first string diagonally across the center of it, face up. Lay the next string on top of the first one, right sides together. Sew with 1/4" seam. Flip second string open, press (by iron or fingers). Continue across the foundation. When one diagonal is complete, turn the foundation around and complete the other diagonal. Progressively shorter strings can be used as you approach the corners. When the foundation is completely covered, turn the whole block over and trim the edges even to the foundation piece. Stings can be scrappy, or applied in an organized manner according to a color plan. Width of the strings doesn't matter. Random widths add interest.
Hope that this helps!
Sue