As you may know, since April 1st, 2017, the payment of advances for VAT quarterly declarations has been abolished. This to simplify the administration of VAT legislation. However, you will have to pay a December advance (if any), as is usual for VAT monthly declarations.

The December advance is:
1. Either equal to the VAT due in the period from October 1st to December 20th,
– If payable VAT > deductible VAT: You fill in the difference in grid 91 (to be paid before December 24th).
– If payable VAT ≤ deductible VAT: You fill in 0,00 in grid 91, you do not have to pay an advance.
2. Either the VAT you had to pay for the third quarter VAT declaration.
– If this declaration is a payable balance (grid 71): You must pay this amount before December 24th and don’t fill in an amount in grid 91 of the fourth quarter declaration.
– If the third quarter declaration results in grid 71 = 0,00 euro or grid 72 (credit): You do not have to pay an advance and don’t fill in 0,00 in lane 91 of the fourth quarter declaration.

You can choose which system you apply.

Source: https://financien.belgium.be/nl/ondernemingen/btw/aangifte/periodieke_aangifte/verplichte_voorschotten