Let's define some parameters:
L: loan amount = $80,000-$12,000 = $68,000
i: periodic interest rate = 0.9167% per month
n: number of payments = 15*12 = 180
P: level monthly payment
The next step is to calculate the level monthly payment using this formula:
Plugging in the values above we get $772.89
To build a table:
The interest portion of the payment is the prior balance times i.
The principal portion is the difference between the payment and the interest portion.
You can see an example of this here.
For the formula method:
Let D: the balance after kth payment (k=12*12=144)
D = P[1-(1+i)^-(n-k)]/i
Plugging in we get $23,607.70