Step 1 ~ Know Her Size

If you don't know, you could ask her. You don't have to tell her what it is for or you could ask her close friend or relative who can snoop without arousing suspicion, they'll enjoy keeping the secret!

For the more adventurous take a peek in the wardrobe and through her lingerie drawer. Find her most comfortable everyday bra and look in the back for the size then check our size chart

For corsets it's a little more difficult, you may be better off asking your partner - see corset advise