This is a huge and poorly drawn image but I had to fit in everything!
#1 is for the halters. I made them all with two layers of fabric. I think this is the best way because it's nice to have thicker fabric and the edges all look good. The red lines are where you should sew first, right sides together! The front and back will kind of be pockets with the sides still unsewn. Here is where you should add ribbon or lace or whatever to the front, so the edges will get pulled into the side seams.
Then turn inside out and sew (right sides in!) to the back
The elastic-back version is a little tricky because I found it hard to get the elastic to line up vertically.
#2 is for the denim skirts. I used those funny shaped pieces 8 for a skirt, sewed em all together into their own skirt shape, then attached the waistband. The waistband was a double layer tube. You can cut off the bottom of a small t-shirt if it fits your waist.
An easy way to get the waistband on is to leave the main skirt part right side out, and then turn the waistband right side IN and slide the waistband over the skirt. Then you can sew along the top.
#3 is for the puffy lace skirts. I used large squares (I basically cut the sleeves and collar off of t-shirts). with a SMALL hole in the middle and a small "channel" to the middle hole. The hole needs to be WAY smaller than you think. The circumference of TWO of the hole should equal your waist size.
You need 2 of those square things. The straight edges of the "channel" get sewn together and the two holes make one hole. Is this confusing? I can draw it out better.
For the lace, I just put it on top of the t-shirt material and treated it as one piece of fabric. I attached the waistband using the same method as #2.