Start with an overly sized shirt (I got one of my dads old ones, size XL) Solid colored shirts work the best, shirts with graphics I found look weird bc u are turning the shirt upside down it will look bad.
Next, turn the shirt upside down and stick your legs through the head hole, try wriggling you legs to stretch out the hole if its to tight. (sorry for my messy room in the background lol)
Take the sleeves and tie them together in a double knot.
Take off the shirt and lay it back down on your flat surface, at the top of the shirt above the tie draw a circle make it medium size because the shirt WILL stretch
Now take your scissors and cut the circle out, try the shirt on and make sure your head fits through it, then take it back off
Now at the top of your head hole, cut the middle part for your halter
Now put the shirt back on and tie the two flaps together in the back (sorry for my straps the shirt looks better in a strapless bra) and your done! enjoy :)