Take an oversized tshirt and cut where I have marked in dark blue. Cut a V from the front of the shirt, and a half circle from the back of the shirt.
Also cut a band from the bottom so that the tshirt is a little shorter than the final length you would like it to be.
If you have a larger chest you will most likely need to add darts to make it fit better, so cut off the sleeves.
Use a zigzag stitch around the edges of all areas you cut off or a serger to prevent it from unraveling. Then fold over the V & half circle neckline and hem all around using a straight stitch.
Take the band you cut from the bottom and hem it to be the width of your hips or where you would like the tshirt to end.
If you are going to add darts, fold over a triangle shape where I have drawn in the gray. Cut a rectangle out of the sleeve as I showed in the first photo (in gray) that's equal to the widest part of the dart. Then reattach the sleeve.
To attach the bottom band to the tshirt, make several darts across the bottom of the shirt so that the large shirt is the same size as the bottom band.
If you would like you could also use a gathering stitch all the way around to scrunch the shirt smaller.
And, you're all done!