ive had to do that for some shirts and dresses too!
alright heres my solution...
take a bra that fits you and holds everything up
cut off the side straps that go around your back but leaving about and inch on either side
then fold the remaining inch over once nad then again and sew...it will give it more support later on...
then cut off the straps that go over your shoulders where the red line is below
if your bra does not have a seam at the top of the strap, fold it over and sew one.
now get a few hook and eyes... youll need about 6-8 hooks and 6-8 eyes times how many backless shirts you have
take 2-3 hooks and place them on the OUTSIDE of the side of you bra facing away from the material like belowimportant! make sure all hooks are facing outwards!
then place one hook at the top of the strap facing outwards and back yet again
then, this is where it get kinda tricky...
basically you are going to put the eyes of the hook and eyes on the shirts you need the backless bra for... so put the shirt on while holding the bra on and mark where a good place to place these would be so you will have support and the bra wil be tight aginst you (so it stays on...thats the point of this after all!)
after you place the hooks on the shirts, attach the bra and wear the shirt!
tada insta-backless bra!!!
if you need any more help with this just email me because im not sure how to send or check messages on here just yet
hope this solves the problem!