You need to gather the front skirt portion to fit onto the front bodice. Sew or serge that gathered skirt panel to the front bodice pieces. Press seam. You may topstitch if you wish.

Those funky lines are my gathering threads. If you need help gathering check out the tutorial here on SM!
Now we're going to take the back panel and sew the small rectangle you cut out onto this piece. We'll call this rectangle "A". You should've cut a slit 2.5" down the center of your back skirt panel. If you didn't do that, do it. NOW
Press "A" in half lengthwise and press in all four edges using a VERY tiny seam allowance (like 1/8")

Now you're going to SEW "A" to the slit. Right side of "A" to right side of skirt panel.

We're upside down here, but get the point?
Fold "A" WST and sew along the slit again.

Now fold "A" in half upways and sew VERY close to the bottom edge.

and closer

wow, blurry. Sorry.
Now, gather each side of your back skirt panel. You'll be gathering two seperate pieces in essence. Gather it to fit each back bodice piece and then sew or serge the skirt panels onto coordinating bodice pieces.

See? Two seperate gathers.

Gather to fit.

Try to catch those flaps in there.

We should have something like this. You shouldn't have sewn the placket down.
So this is confusing, but you should have one part of the placket in with your back skirt. The other part should just be rocking out.

Hopefully this picture makes it clear.
Now, we're going to sew or serge the sleeves onto the bodice. Do both sleeves. If you're doing the sleeveless version, skip to step 28.

That's what you should have pre-sleeves.
Turn the sleeves up 1/2" and press then turn up again, press. Topstitch the sleeve hem. Do this to both sleeves.
If you lay your dress out you'll have kind of a plus sign.

Fold the dress RST and sew or serge up each side. One side then the other. Start at the bottom of the skirt and finish at the sleeve that way you can lock your serging in better. One side then the other. If you're doing the sleeveless version you'll just finish up at the armpit area.
Now turn your skirt hem 3/4" and press. Turn up again and press. Topstitch all the way around the skirt hem.
Now sew on your buttons, or place your snaps and voila! Maddy's dress!!!