Lay out your old t-shirt
Hold your t-shirt up against you upside down so the hem is facing up and mark how long you want your bandeau to be(I used a pink marker incase I messed up)
Cut it off
Put it on with the hem at the top and pull tightly to the left or right to see where to cut
Make sure to make it really tight because the shirt material stretches and bandeaus are meant to be tight to keep all that needs to be in, in. (if you catch my drift)
Mark the 2 edges when you pulled really right that you're going to cut to make it smaller (I'll put a video in if a lot of people don't understand)
Cut the space between those two marks off(the lower piece in the picture)
Sew the two open edges together and you're done!