Short answer. From the undisposed part of the estate first. Article 855 requires the omitted child's share to be taken from the part of the estate not disposed of by the will, and only where that is insufficient is the balance taken proportionally from the shares of the other compulsory heirs.
What the law says
The share of a child or descendant omitted in a will must first be taken from the part of the estate not disposed of by the will, if any; if that is not sufficient, so much as may be necessary must be taken proportionally from the shares of the other compulsory heirs.
Civil Code, Article 855 — Share of an Omitted Child. Read the full provision →
A queue, not a free choice
The article sets an order and the order is mandatory. The omitted child's share must first be taken from the part of the estate not disposed of by the will, if any, and only if that is not sufficient does anything come out of the other heirs. So the first question in any of these disputes is arithmetical rather than legal: did the will actually dispose of the whole estate? Wills frequently do not. They name specific properties, or dispose of a fraction, or were made before the testator acquired his largest asset, and whatever is left over is the first place the law looks.
"Proportionally" protects the other heirs from each other
Where the undisposed residue does not cover the share, the balance is taken proportionally from the shares of the other compulsory heirs. Proportionally means pro rata: an heir who was given half the estate contributes twice as much as one given a quarter, and no single heir can be made to carry the whole reduction because his property is the easiest to sell or the most recently transferred. It also means nobody is wiped out to satisfy the claim. Each share is scaled back by the same proportion, so the pattern the testator chose survives in reduced form.
It is the compulsory heirs who bear it
The article names the source precisely: the shares of the other compulsory heirs. That is a defined class rather than everyone mentioned in the will, so identifying who falls inside it is part of the calculation and not a formality. Working out the figure therefore takes three inputs — the value of the net estate, the size of the legitime the omitted child is entitled to, and how much of the estate the will left untouched. Get an inventory and a valuation before anyone proposes a number, because every step of this depends on the first one.
This article answers only part of the problem
Article 855 tells you where an omitted child's share comes from. It does not decide the separate and often more consequential question of what the omission does to the will itself, which is governed by other provisions and can, in some situations, unsettle the institution of heirs rather than merely adjust the arithmetic. So do not treat a proportional reduction as the automatic answer. Establish first whether the child was omitted from the will entirely or merely given less than the law reserves, because the two situations are not treated alike.