A long, long time ago, I promised a lesson in making lingerie bows. Not that I know how anybody else does it. This is just how I worked it out.
First up, thread a needle.
Take a length of ribbon longer than you need, at least 10 cm long. Hold it like so.
Flatten the loop down on top of the criss-cross bit, sitting just a bit higher than the criss-cross bit. Take your threaded needle and stitch a running stitch, starting at the bottom of the bow and heading towards the top.
Pull the thread tight to scrunch the bow down a bit.
Wrap the thread tightly around the centre of the bow, forming the cute bow shape. Stitch a few stitches to the bow to secure the thread, but don't cut it off yet.
Trim the ribbon to the length you like. I like to snip the ribbon ends in a vertical line, lining up with the outer edges of the bow. Note that the needle and thread are still attached.
Attach the bow to the front of your bra, using the attached needle and thread. This saves you having to re-thread your needle. Secure and snip threads.