Well, I've done some basic sewing in my time, (VERY basic) I've made what is called in the ren-faire circles as a T-Tunic, but what you want has to have a hood, which is beyond my skill.
To create a robe with the long hanging sleeves and the hood is going to take a good deal of time and effort, not to mention you may need more than two sheets. (are we talking about bed sheets?)
Are you determined to make the robe yourself, or is there something really different you want to do? If not, the Grim Reaper is a REALLY popular costume and you can get really good quality robes with hoods on the net. As a matter of fact, typing "grim reaper" in the apparel section of Amazon.com came up with a ton of em:
If you really want to do it yourself, and you're able to do basic sewing, go to Wal-mart or some place that carries fabric and look for Simplicity Design # 5840
and make a couple of changes:
If possible, exagerate the hood (make it longer, wider...all that really has to follow the design is where it attached to the cloak),
Also make the sleave cuffs larger. A sleeve is basically a triangle with the sides sewn together to make a cone. Leave the end that attaches to the cloak the same, but make the cuff end longer, and you have wide cuffs)
and if you'd like, sew up the front seam (or add hooks or buttons).
Thanks everyone i went out and bought 3metres of black fabric for 3pound from my local market and am going to make a basic robe i have a sewing machine and know how to basic stitch. I am going to do a basic tunic shape like urshag suggested and make a simple hood and attatch them. Thanks again for your help everyone.