In general you use terms that the attorney preparing your will knows how to craft.
Some of your accounts allow you to specify beneficiaries. These designations generally overrule the instructions in the will. This also applies to insurance policies, retirement plans and the like. They generally allow you to specify percentages if there are multiple people who will receive the proceeds.
If you are designating that child 1 and child 2 will split the money in the checking/savings account, don't forget to update the will or account specific beneficiary list after the third child is born. The will/beneficiary list can be written to not require a listing of the children by making it clear that it applies to all the children.
The laws of your country/state may also require that specific accounts be designated for a spouse unless they agree to give up their rights. The lawyer will know the laws.
It is a good idea to review and update the will after marriages, divorces and the like. It prevents surprises many years later when spouse #3 finds out that the bulk of the money is going to spouse #1 and the children of spouse #2.
Many people suggest the creation of a death book, that will designate the important information regarding accounts, policies, and benefits. It can also include the most recent version of the will.