Hi Crystal Ball Gazer,
If you make subsequent amendments to your will, you can do so either on the will itself or by a codicil (amendment document).
1. will itself: you will need to make sure that such amendments are signed (or initialed) by yourself in the presence of or later acknowledged by two witnesses (who cannot be beneficiaries under the will) and dated. Such amendments must be made before the execution of the will (i.e. finalising of the original will).
2. codicil: this document needs to refer to the original will, expressly mention that it was meant to amend the original will and in all other respects (i.e. where it is not inconsistent with the original will) it reaffirms the original will, and follow all the formalities required for a will (signature of testator/will-maker, two witnesses who cannot be beneficiaries under the will, dated, in writing, attestation clause). You can draft this yourself but it is best to have a lawyer draft it up so that you can be sure it is valid.