A darning stitch is just like you would use to darn a hole in a sock....I,personally, make lines of stitches, fairly close together, all running in the same direction...these go beyond the weak parts....and you've placed the underfabric also beyond the edges of the weak part.. Then, do lines of small stitches running at right angles from the first lines....it's a sort of weaving, and as you are going through both the old and the new, you are adding strength to the old fabric.