Make a shirt template from a nicely fitting shirt or dress. You can use a piece of yesterdays newspaper for that. ^^

Draw the shape to the oversized shirt. line up the highest point almost to the shoulder seam ( the remaining bit it seam allowance, oh and take care not to mess with the neck hem of the shirt ), this way you can make the shirt longer on the bottom.

Depending on how long your template and your shirt are, freely draw the bottom, angled like I did, v-shaped, u-shaped, straight, whatever.

Pin the front and back together, ten cut out with seam allowance. Make sure you cut both layers evenly.

From the remaining side parts of the shirt, cut two strips ( about 1 inch wide, lenght is up to you ), grabb one end with each hand and just pull. The strip will roll up and stretch aaand form your neck holder. :)

Decide which side is gonna be the back and cut off the top part in a slightly round shape. Make REALLY sure you don't accidentally cut the front, too!!!

Just hem top and bottom seam ( pin if needed ), adding the straps to the front

You're actually finished.
Ohsosimple! :)

I am gonna make more of these cause they're so easy to make and they are a great base for further recon, plus they are pretty cheap cause the ogitinal shirts are from a dollar store. ;)