I cut the top to size and decided to make a tulip hem dress instead of one that was just gathered at the top.
For this, I cut my material double the circumference of the waist and then folded the material in half and cut off an arc at the bottom of the skirt through both bits of material.
To do this, I pinned it on the right sid of the material, then folded the material to the inside to make a hem, so that only the bottom wavy bit showed on the right side of the dress.
I then changed my sewing machine setting to the loosest stich setting and sewed a row across the width of the skirt material at the top where the t-shirt will be attached leaving a long thread at the beginning and end so I had lots of space for size adjustment.
I then gathered the skirt from one side, gradually moving the bunching material along until I had gathered the skirt to be the same length as the t-shirt opening.
I then attached the skirt and top together by stitching round with right sides facing, making sure that the tulip part of the dress would be at the front of the dress to one side.
Here it is finished and a very unreluctant model modelling it!!