Firstly, the picture on the pattern shows you attaching from the right first, I couldn't comprehend sewing this way so I started on the left.
Starting in the middle, first don't forget to snip the middle of the neck, this helps A LOT. you need to be careful not to go too far, but you can snip a little and cut a but more away later. on the top bodice piece identify the middle of the point, 5/8 in from the point (luckily for me, this is about at that dot) then pin this exact point 5/8 from the center of the lower bodice under the snip. use the pin to mark out that spot (you could make a mark using tailors chalk or similar). Pin the top and bottom bodice pieces together at the edges working from the center to the left edge.
Now, from the left edge, sew towards the center 5/8 from the edge until you get to the pin/marker in the middle of the neckline. Make sure the needle is down and lift the presser foot.
You now need to pivot the whole lot, so the bottom bodice is lined up with the 5/8 marker on your machine ready to sew. This is where the snip comes in handy. If you find this difficult to pivot then try making the snip slightly longer, but not too close to the needle. Then, you need to pivot the top bodice piece so it lines up. You will notice it gathers a lot in the middle (see above) that's fine. At this point I found it easier to pin by lining up the right hand armhole edge and working in towards the needle in the center. Make sure that there is no fabric being pinched together under the needle (just give it a quick pull/stretch with your hand) pop the foot down and sew to the other edge.
You can see that there is a lot of the top bodice fabric gathered in the middle underneath but this doesn't matter as it will all get tidied up when you trim the seam allowance and top stitch it down later on.
And fingers crossed you end up with a nice, smooth, unpuckered sweetheart neckline. Now give yourself a pat on the back, breathe, make a cuppa, and get on with the rest of the pattern.