It varies by card. A lot of times you'll see a 0% offer that is only good upon app, but later you can BT for 1.99% or 3.99% or whatever.
A lot of sites let you do it online, although you might have to do a lot of clicking before you find the actual option. Otherwise, call customer service.
General word of warning to other readers: never, ever do a BT or other low interest transaction on a card that already has a balance, and never charge anything on a card that already has one of these. Usually (not always), the CCC will apply all payments to the lowest-interest charge, and that $45 restaurant tab will just sit there, chuckling away, drawing 13.99% APR or whatever your usual purchase rate is, until the lower-interest stuff is completely paid off.
The CCC really, really hopes that you're not aware of this, or that you'll get careless. This, along with the frequently forgotten BT fee, is how they make money off of balance transfers.
* Credit is a wonderful servant, but a terrible master. * Who's the boss --you or your credit?
FICO's: EQ 781 - TU 793 - EX 779 (from PSECU) - Done credit hunting; having fun with credit gardening. - EQ 590 on 5/14/2007