I think because she is so old, I would do it very gently...
Tracy describes how to deal with that problem in the toddler book...
Okay...
1. Breastfeed as usual but then take her off the breast before she is asleep and get the lovey instead
2. Rock her to sleep in a rocking chair or something like that
3. Next day don't breastfeed before tuck in, just cuddle her to sleep
4. Next days cuddle and rock her until she is nearly asleep and then put her in cot and gently pat her back and say shhh shhh until she is asleep...
The whole thing might take a few weeks but it is a common problem. If you have always nursed her to sleep every time that means also that she has got a lot of trust in you and that you have been very consistant. Just do not let her cry and alone in the cot, that would be horrible... She has now learned that she can only go to sleep with a breast and it is time to teach her inependant sleep. Because she is already 13 months, a lovey could really help (like a teddy or a blankey or something that smells of you)...
Good luck and HUGS
Steffi
P.S. I haven't been through that so there will be much better advise than mine but I just read the passage in Tracy's book about exactly that