Pick out an oversized T-shirt that's long enough on your body to wear as a mini-dress. Choose a shirt in a color and design you find appealing.
Cut the T-shirt apart at the armhole seams, removing the sleeves.
Put the T-shirt on and have one of your friends help you create an entirely new, fashionable off-the-shoulder neckline. Draw the neckline directly on the shirt with a fabric pencil, making a diagonal line that starts at the inner side of one shoulder seam and ends at the upper part of the side seam on the opposite side of your body.
Cut along the new neckline to remove one of the shoulders of the T-shirt. If the T-shirt is too big for you at the top and doesn't stay up, put the shirt on inside-out. Pinch the side seams in until the shirt fits you, then pin the seams in place.
Fix the side seams by sewing along the areas you've pinned. Trim the seam allowances to 1/4 inch.
Fold the neckline and armhole edge 1/2 inch to the inside of the dress and pin the edges down. Sew along the folds.
Iron the new neckline flat; wear your new one-shoulder dress belted or loose and fashionably free.