In short - at any time between the BEGIN TRAN and the COMMIT.

So -

BEGIN TRAN

(do whatever you need to do)

(test that it did what you want)

COMMIT TRAN

or

ROLLBACK

Keep in mind that you will likely be keeping some things locked/unavailable while you're in the transaction, so don't take overly long so as to not disrupt the regular flow of data.

You should also do some reading on "Isolation levels" so as to determine what stays locked or should stay locked during your transection.