Fold over sides of both top pieces and topstitch
Lay one top piece over the other to the place where the arrow points, pin them together then gather the lower edges of both
Pull gathering threads until the top pieces fit between the two arrows
marked on the midriff piece
Stitch top pieces inside doubled layers of midriff, leaving an opening where the top pieces are, through which you're going to turn the midriff. Clip corners and turn.
Sew skirt side seams, then stitch skirt to midriff, leaving back edges of skirt a little wider than midriff
Then fold those over and stitch center backs and hemline
Sew on velcro or snaps for closure