Get a shirt like this.
Well a regular long sleeve shirt would work.
You can even use just a regular t-shirt.

Cut from armpit to armpit

Cut off the sleeves and make four matching strips.
If you want the skirt to be on your lower hips, cut six strips

Take two of them and attach them together to make a strap.
The same with the other to strips

At this point, or before, you might want to make the skirt smaller.
I did this by adding a drawstring, but you can use pretty much any method

Attach the straps to the back.
On the front, you can add buttons, use velcro, or just sew them on.

And then your done