Don't worry if there's a dumb design on the shirt. You can sew it so the lame-ness is on the inside :) The grey shirt i used was some old adidas work out shirt of my dads but you cant even tell!!
Put the tank top on and find the smallest part of your waist.
Mark the tank top where your waist is and take off the tank top
Cut horizontally across
This is what it should look like when its cut. The top will be the bust of the dress
Find an XL t-shirt and cut off the arms. This will be the skirt
After cutting off the arms, measure 2 inches above the arm holes.
Cut horizontally across again.
Put the skirt on over yourself. The top should be a bit higher than the smallest part of your waist.
Find the largest part of your hips and measure how much excess you have on both sides (my own was an excess of 2 inches on each side).
Take off the skirt and lay flat. Measure in how ever many inches you needed in from the edges (for mine in measured in 2 inches).
Mark the skirt and pin.
Repeat on both sides
Sew along this mark on both sides
Lay the bust of the tank above the skirt to measure how slanted you need to make your waist. Dont worry if yours looks like mine. It looks like i have the tiniest waist and the biggest hips but you have to remember that you have an butt!!! So its okay if the slant looks extreme!!!
I missed taking pictures of the next
Some how i missed taking pictures of this step...
but you just sew along the silhouette of your waist and hips....
Flip the skirt inside-out and try it to see if where you need to go in more.
Continue sewing and trying it on until its truly a custom fit.
Line up the bust to the skirt
Make sure the two are completely matched up. Check the side-stitches to make sure :)
Pin all the way around the waist
Make sure your pinning/sewing the inside
(Close up :D)
-Inside out view-
Sew along the waist
Cut off extra fabric and flip it right side out :)
Add a belt, some buttons, a zipper, or some ribbons for an extra Oomph!!!!