DIY: Off the Shoulder Top

Off-the-shoulder looks are in full swing for summer, so here's a tutorial for a popular style.
My roommate first sent me a picture of this top that she wanted from Tibi.

The original is a few hundred dollars, so I took some time to figure out the two real questions. Where do the armpits go? And how can I begin and end the elastic parts easily? After a few weeks, I finally had the courage to do this project, with some marble fabric I found.

You will need:

1 yd of material (polyester/rayon, light cotton, or light linen recommended)
1 yd of 1" wide elastic, preferably the soft comfy kind used for underwear

Here are the steps…

Cut out a front and back rectangle (mine were approximately 24" x 18" each)
Fold the front rectangle in half and taper the top edge to be 21" wide from 24"
Cut two sleeve rectangles (mine were approximately 17" x 14")
Cut a long strip for the top casing (mine was approximately 5" x 50"
Sew the front to the back along the two short sides, right sides together, with a straight stitch
Fold each sleeve in half, right sides together, and seal the short edge with a straight stitch
Flip the body and sleeves to all be right side on the outside
Pin the sleeves to the body at the top of the shirt, seams aligned with seams
Add two more pins on each sleeve to further attach its inner side to the body, 2.5" away from the seam pin on each side
Sew a short straight stitch on each of these 4 pins from Step 9 and remove the pins, the sleeves should now each have an inner side attached to the body
Cut away a U shape on the two armpits of the body and the inner side of each sleeve, the U shape should start and end between the short stitches, go 5" down, then round out at the bottom to connect
Sew the armpit of the body to the inner side of each sleeve along the cutout U shape with a straight stitch
Fold one raw edge of the long strip and pin it to the top edge of the shirt, right sides together
Pin the long strip to the shirt all the way around and overlap where it began
At the two front seams where the sleeves meet the body, pin in a 0.5" fold to the long strip that points outward in opposite directions (see video for visual on this)
Sew the long strip to the shirt all the way around
Add a straight stitch to the two small folds to secure them (see video for visual on this)
Cut down the elastic to the correct length, it should reach comfortably around the back of your body, but be able to stretch to the full length of the gathered part of the casing (mine was 27", later adjusted to 22")
Sew the two ends of the elastic to the small folds with a zigzag stitch so that it encircles the sleeves and back of the top
Tuck the free edge of the casing in 0.5", then fold it down again to enclose the elastic
At the same time, push the raw edges between the casing and top upwards and hide this into the casing as you pin your way around
Sew the casing in place with a straight stitch, all the way around, stay close to the edge, and do not sew on the elastic
Hem the sleeves and shirt bottom by folding the edge inward twice and sealing with a straight stitch