This top isn't TOO hard to make, as long as you work carefully.
1. Take a tank top (preferably a loose one, like in your pictures) and trace around it to make a pattern. The front has a low, round neckline, and the back has a low v-neck.
2. Cut your BACK pattern piece off at the waist line, and put the bottom piece aside for now. In the top half, draw a vertical line at the centre back - to the point of the v-neck.
Now you need to divide the back into equal portions - maybe a bit over an inch wide? You'll need about six sections, but don't go into the arm-hole, or the top of the shoulder.
Cut out each of these strips - it may help to number them, so you know where they belong, and which way up they go!
3. Fan out your pieces as equally spaced as possible, keeping the shortest (centre) pieces together and vertical. Make sure the bottom corners of each strip touches its neighbour.
Do all of this on top of a new sheet of paper, so that when you are happy with the placement, you can carefully trace around it.
As you can see, the edges are a bit wonky, so you need to trace a new v-neck shape - it's actually gently curved, but this will straighten out when you make the garment. You also need to square off the bottom edge. These new lines are purple on the diagram.
4. Once you've done all this, re-attach the bottom half of the back panel.
5. You also need to construct the bottom waistband bit. Measure the bottom hem of the front and back panels, and draw a rectangle of this length. It needs to be twice as deep as you want the finished version to be.
6. Cut out all your fabric pieces carefully.
7. CONSTRUCTING THE BACK PANEL: All of that excess fabric needs to be gathered in. Sew a line of stitching close to the v-neck edge (do one side, and then the other, leaving long threads at the start and finish), using your longest stitch length (or by hand). Don't back-stitch! Place a pin perpendicular to the stitching at one end, and wrap the thread ends securely in a figure-8 pattern. Then, pulling gently on the bobbin thread at the other end, gather up the fabric til that edge returns to the size of the original v-neck. This will need to be "finished" - use bias tape, or hide it with a strip of your fabric.
8. Sew your front and back panels together, and sew on the waistband... I can't really tell what's happening on the shoulder, but you should be able to make up some extra pieces, and pleat/scrunch/fold as you like, and attach them.
Hope this SUPER long post has given you enough help! I haven't covered every little step, as I'm not sure what your skill level/experience is, but feel free to come up with questions!