You should have eight pieces, two top fronts, two top backs, bottom front, bottom back and two sleeves.
To gather, sew a basting (long) stitch at 1/8 of an inch on each sleeve, the bottom front and bottom back. Be sure to leave plenty of thread at the end so that you can pull the threads in order to gather. Pull the threads for each sleeve and each bottom piece.
All seams will be 1/4 of an inch unless otherwise stated. If you are using a fabric that ravels you will want to use the serger or serger stich within the seam allowance.
Next pin the first top front to the first top back at the shoulders, right sides together and sew each shoulder. Repeat with the second top front and top back. Next iron the seams so that they are flat. Pin first top front and back piece to the second right sides and fronts together. Sew the necklines. Clip around the edge of the neckline seam (this is the part closer to the neck when it is inside out).
Fold with the right side out and iron. Sew a basting stitch at the edge (it can be less than a 1/ inch seam.)
Pin the gathered front to the top front so that the bottom seam of the top and the top seam of the bottom match. (Right now, the right sides should be together, so that when you are done sewing, the bottom part of the bottom piece will be near to the ground). Smooth out so that edges of the top piece and bottom piece (as mentioned above) match. Smooth gathers so that they are even. Finish pinning and sew. Repeat this step with the top and bottom back, ironing each seam as you finish it, so that everything lays smoothly.
Next, pin in the sleeve and smooth gathers so that they are even. (Right sides should be together on this too that way seams are at the inside of the nightgown.) Finish pinning and sew. Repeat with the second sleeve. Iron each seam.
Turn nightgown inside out. Match edges, pin and sew. Turn it right side out. Pin sleeve hems and bottom hem. If you want to add lace, this is the time to do it. Sew hems. Iron nightgown, clip threads, wear and enjoy.