You will get a membership number. Go to BB and create an account and add that number.
If you already have a BB account you may or may not be able to add it and even if it looks like you did add it, it may not stick.
It may be easier to just create a new account.
When you buy stuff, the discount won't show up until the last page before you hit submit.
If the discount does not show up, see above and create a new account.
The discount does not stack and doesn't work with sale items. It may in store but YMMV depending on SA.
If you have an amex, sign up for free shoprunner to get 2nd day ship on many items.