Work out the total cost
Add the transfer fee, any annual fee and the rate that applies after the promotional period, then compare that with what you pay now.
Set a repayment that clears it in time
Divide the transferred balance by the months in the promotional period and automate that repayment.
Don't use the new card for purchases
New purchases can attract a higher rate and may not get interest-free days while the transferred balance remains.
Close the old card
Pay it to $0, redirect any direct debits and cancel it.
Diarise the end date
Set a calendar reminder a few months before the promotional rate ends.