First of all, don't forget to check the "separate 13th salary" box in the company's settings:
Calculate the month of December as as any other month.
Print pay slips etc.... Make the payments. Make the voucher if you want to make them independently (1 for the month, 1 for the 13th) otherwise it will also be possible to make a consolidated one.
Close the month as usual.
Then go to separate 13th salary :
There is no "Initialise" in this case. Click on Compute all ans click on Yes to use the static data :
Print pay slips etc.... Make the payments. Make the voucher if you want to make them independently (1 for the month, 1 for the 13th) otherwise it will also be possible to make a consolidated one.
Close the month of the 13th separate salary.