Okay so first step, you want to cut a tube top but cut the back a little lower than the front otherwise you'll have some problems later with the fabric that simply gathers.

Then sew and pull on the thread at the front with a needle (don't use your sewing machine) to make it look like this

Or you can sew a piece of elastic on the back to make it look like fabric gathers there but it's a little hard. The results are the same anyway and I try to keep things simple.

Now cut some straps and add then to the top.

Now the fun part, making the skirt to stick it to the top! :D

For this, I will be referring to threadbangers video on how to make a circle skirt. Just jump to 5:10 minutes.

http://www.youtube.com/watch?v=CQ05GbEs2Po

OH and yeah! you don't need to make a zipper and stuff, just leave it as a circle skirt.

The hard part now, sew the skirt to the top and you should be done.