So how do you do this?

1. Get a pair of men's briefs that have a waist size that matches the size of your bra.
I.E. I'm a 36 bra so I get a Medium in men's undies.
No tricks.
No math.

You'll recognize this as the front section of the undies (or maybe not, I didn't provide a lesson in the make up of men's undies, trust me, this is the front where the plums are smuggled) which will now be the back of your bra..
For an average bra cut along the pink lines I've shown.
Don't cut into the comfy leg seams, They are nice and padded and make good bra straps.

Keep cutting around the back (now your front)

You can set down your scissors and throw on your new bra or you can serge the edges or turn them over and stitch them down. Its all personal preference or depending on how much of a hurry you are in, as the edges won't fray anyway.

You can also embelish it with your choice of fun embroideries or stamps or tie dye it.

There are several other ones you can do.
Use your imagination.

For Example:
V-neck bra

Cut again along the lines - leave the straps.
cut the back (crotch) the same as above.

Back less bra for a halter top.

This the back (crotch) you are cutting out most of it.
note that the lime greeen marks where you will actually be cutting the straps/leg seams this time
Cut the front the same as one of the above examples
Stitch the leg seams together to form a halter strap.

You can also completely cut the straps off and stitch the "bra" to the inside of your top.

Need undies?

This is just too cool! And WOW are they comfortable!