The red lines show where to cut in the first place. You don't HAVE to cut the sleeve and neck seams, of course, but I highly recommend to cut off the bottom seam for equal outcomes.

I did cut the neckline to a neat V-neck, leaving the back of the neck straight.

Flip the whole thing over, mark the middle line with chalk or something similar and draw straight, equal lines to the sides. Decide on how long and thick you want those, it's totally up to your taste.
That's where you are going to cut. Another quick prayer of panic might be necessary.
You might wanna experiment on an old shirt first?!?

Make the hell sure you only cut the back of the shirt and let the front unharmed

Kat seems to stop before the neck seam, but I like to cut through and let the upper strands hang down. Otherwise, if you're heavily changing the size of the shirt, it might look kinda weird.

Now that you've got the back all messed up, you can decide weather to leave the straps as they are and start knotting ( that's ok if you want the shirt to be smaller than before ), or if you wanna pull them so they form nice, round strings which is perfect for making the shirt bigger cause you're gaining length.

Get someone to fit the shirt onto your body. Making this yourself might get a desaster cause you just can't see your back and you might get confused with all those dangling strings.

I made my mother do it, telling her where to pull tight and not to, plus I asked her to take two strings from every side for each knot. Go wild, use 1, 2, 3, 4 at a time.

Make final adjustments as needed, snip off the access strings if preferred, get dressed and go get the compliments you deserve.
Before, take pictures and show me. :D

Of course these look best without a bra, but there are girls who just can't get out without wearing one. :(

I went clubbing in this one and all the girls were like WHOOOOAOMGWHEREDIDYOUGETTHIS??? :D