We are creating this top.
For construction, use an appropriate stitch for stretch fabric. There are several options here, and it's up to you which one you use, but I'm not going into full detail here. Do a test swatch and check you're happy with it. You certainly don't need a serger to make this up.
If the t-shirt has darts in the front, as mine does, these need to be sewn first.
Then the two front pieces are connected across the top edge, and along the centre front line between the bottom edge and the mark I put for the neckline. For the centre front seam, use a larger, 2cm seam allowance to have a generous fold. [3/4"]. That's the orange lines on this diagram.
Your neckline will be straight across, I modified mine later.
Press this fold outwards, continuing up the unstitched part to create the neckline.
From here, the next step is to a gather across the front.
At cutting out stage, we drew in a chalk line. This line needs to include the full measurement of the extension, and an additional space on each side. My original extension was 15cm. I now increase that by 4 - 5cm (2") on each side, and put a mark at each end.
To construct the knotted layer, begin by finding the centre point of the trapezium, on the short side.
Mark and clip a point 3cm [1 1/8th"] to each side. My picture here has 2cm each side, but it was a bit snug so I'd go larger next time.
Hem or finish both of the long edges. (No photo, sorry. I forgot).