This is a very simple adaption to The Slip Dress and the effect is a looser more casual style.
Take the skirt pattern piece and add 4cm to the ‘Place on Fold' edge or centre back (A). Mark the original centre back position with chalk or a small snip. Cut out the skirt piece.
With the right side together stitch down where the centre back is by 1cm (B). Open out, line up the folded edge line with the centre back, press, then stitch across the top as shown in (C) wrong side and (D) right side.
Make up the dress as stated in the instructions.
This simple pleat addition could also be added to the front for even more fullness.
In the sample shown here, the fabric is from Til The Sun Goes Down trimmed with vintage lace bought at a market.