The sack dress, originally developed in the 1950s by Givenchy, has come back into style in recent years in the form of a loose, shift-like shape that resembles the sheath and shift dresses of the 1960s. Identified by its lack of waist definition, sack dresses are basically shaped like sacks with holes cut for the arms, head and legs. In this type of dress, all innovation usually lies at the neckline, which can be made in any style.
Find a loose T-shirt in your size, that has the fit you would like your sack dress to have. You will use this as your pattern. If the T-shirt is made of stretchy material, you will have to also purchase fabric in stretchy material, or else the fit will not be correct. Put the T-shirt on your body, then determine how much longer the shirt would have to be to reach the dress length you would like. Take note of that measurement.
Fold your fabric in half, so that you have a double layer. Lay the T-shirt flat on top of the fabric with the sleeves folded back.
Take your fabric pencil and trace around the sides and neckline of the shirt. On the bottom, add the length measurement you determined in Step 1 and extend the line of the T-shirt. Remove the T-shirt from your fabric. Draw around your lines again, adding 1 inch to the sides, sleeves, shoulders, collar and hem. Cut the fabric so that you have two pieces: a front and a back.
Stitch the front and back of the dress together at the shoulders, with the right sides together. Iron the seams flat and trim them.
Stitch the dress together at the side seams, then iron and trim the seam allowances.
Turn up the hem 1/2-inch and iron it flat. Then turn it up another 1/2-inch and also iron that. Stitch the folded hem in place.
At the collar, turn the fabric under 1/2-inch, just like you did for the hem. Then iron it and turn it under another 1/2-inch. Iron the second fold. Stitch the collar in place.
Fold each sleeve area under 1/4-inch and iron it flat. Fold the sleeve under another 1/4-inch and iron that. Then stitch each sleeve in place. If necessary, hand stitch the sleeves so that they lay perfectly flat.
If your hips are wider than the T-shirt, you will need to add extra width to the bottom of the dress. You can alter the neckline of the dress to make different sack dress designs, or add pockets to the hip area.