Ok first attempt on strech fabric. my trick. I did a running stich where I would normally pin.
I used a little over 1.5 yards for her.
1
Fold fabric so it is 54" wide by 1 yard.
Fold that in half. where the center is cut you neck hole.
2
fold all edges in and do a running stich.
Do the same around the neck.
3
With your machine use a strech stich and follow you running stich.
Try it on, inside out. And mark were you sides meet. Or measure if you want it perfect.
4
Sew up each side you just marked
when you get to the armpit area turn it in a half circle and sew a parallel line about 1/2 inch ( to the outside)
5
cut between the two stich lines
Turn inside out and tada all done.
6
With scraps you can tie a headband.