Start with your over-sized Tee.

Lay your fitted shirt (or tank) inside the tee as high up as it goes. You're only need measurements for the bodice of the tee, so the neckline can go off the edge.

Now cut along the shape of your fitted tee's bodice.

(Upside down pic)

Now cut a sort of triangle whose tip goes in the center of the tee. make one side go off the edge. Wow, I'm bad at explaining that. Just look at the picture.

Now, all with right sides together, you sew. Sew up the edges and sew the top on. The top's two side points should overlap in the front. The rectangle part between the two triangles should go in the back.

(Upside down pic)

Here are the sides sewed up and the top part is going off the bottom edge.

Now cut two strips from your fabric, perpendicular to the grain.

Pull the two strips lengthwise and they should roll up.

Sew these two strips any which way to the tops of the triangles.

(You could also attach the other ends of the straps to the back of the shirt to make a tank top, or criss-cross them to make a racerback. I chose a halter, as it requires the least work!)

And you're done!!!