Place the two skirt pieces together with right sides facing each other. Line up the side seams so that they are even. Gather the top edge by sewing a zig-zag stitch over a gathering cord like crochet cotton or string. Be careful not to stitch into the cord or it will not slide smoothly when pulling up the gathers. Do not pull the gathers up yet.
Machine stitch the hem.
Mark the side seams, center front and center back on the T-shirt hem. It will be divided into four equal parts. You can make a chalk or pencil line or place a pin at those places to mark them. Lay the right side of the skirt to the right side of the T-shirt. Arrange the side seam of the skirt to the side seam marks on the t-shirt.