First of all, don't forget to check the box "separate 13th salary" in the company settings.
After closing November, go to Separate 13th salary :
and click on initialise.
Check the salaries, 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 13th salary the way you would close a normal month.
Go to compute :
There is no "Initialise" in this case. Click on Compute all ans click on Yes to use the static data :