I just looked at Leah Days spider web design, and took off. Did the straight lines from a corner, did one line, went down the side to the center of the side, went across, went to next corner, went diagonally to the next corner, then to the center of the next line
to be clearer try this. Start upper left, diagonally to lower right, go half way straight up, go left to far left side, go down to point even with diagonal line, go right half way, then up to point even with first diagonal line, right from half way point to corner, then finish last diagonal line right to left. This won’t enclose the whole square, go back and do the outer lines if you want them.
Then go back and do the loops from line to line. If you have a large square to put a spider web in, you might want more straight lines, then put curved lines to imitate spider web. Practice a little before trying this. Follow the directions on paper to figure them out first.
Or google spider web free motion quilting and see if you can find a video.
Honestly, I just looked at the picture and took off with the sewing machine. Good luck