I should add a disclaimer that I've never crocheted a wearable item, only smaller stuff!
It would seem to have one row that you need to do with the bind off bits in - these bind off bits are the dividers. After you've done that row with the bind offs, the bits from the edges to the bind offs are the front pieces, and the larger bit between the bind offs is the back.
So - after the bind off row.
Working from the edge to the first bind off is a front. Just work on this bit going back and forward from the edge to the bind off, following the pattern and doing the shaping. You'll finish one front.
On to the next front - presumably need to attach the yarn at the bind off point and work between that and the edge, reversing the shapings to match. You've finished the second front.
Attach yarn and work between the two bind-offs to complete the back according to the pattern.
I think that's what it means by work each body part seperately. Do one front and finish it, do second front and finish it, then do the back.
But to me, the pattern says to do a bit at a time. So you may need to do the row with the bind offs in as one, but after that you work and complete one front only (working from the edge to the bind off bit), then you finish that front after the neck shaping.
Next front piece - presumably you pick it up after the bound off bit, then work that bit reversing the shapings.
Finally - back piece, worked between the bound off bits, has own shaping in pattern.