<- I used a fitted shirt.

If you use a shirt that's a bigger size then your usual you'll get some sort of dress, you'll have more pleats and I think it's more comfortable and pretty.

Grab a shirt.

Cut off sleeves but leave as much usable fabric as you can

Front; Cut along the red line. Cut a straight line from sleeve to sleeve below the lowest part of the neckline ribbing.

Back; Cut a straight line from lowest part of cut off sleeves.

Pin a bias tape onto the part which is indicated blue in the picture (or hem it).

I've made my own bias tape from scrap fabric and used this tutorial

http://www.youtube.com/watch?v=4fEv3VXPUEs

Tunnel piece;
Cut a strip with the same measurements as the neck (orange on picture) but measuring 7cm wide.
press it as if it were a bigger piece of bias tape.
Sew it on, this is going to be the tunnel.

Neck piece;
From scrap fabric cut a strip measuring 160cm long and 7cm wide.
fold it over (160cm x 3.5cm)and sew it. Turn inside out and press. Close both ends.

Pull through tunnel and tie it.