Wikipedia defines a last will and testament as;

In the common law, a will or testament is a document by which a person (the testator) regulates the rights of others over his property or family after death. For the devolution of property not disposed of by will, see inheritance and intestacy. In the strictest sense, "will" is a general term, while "testament" applies only to dispositions of personal property (this distinction is seldom observed).

A legal will can be drawn up on any piece of paper providing it has the following requirements catered for;

You need to be 18+

This isn't true for every state or every country but there will be a minimum age that legislation recognises for a legal will. Some states of the US have lower minimum ages while other countries have much higher standards.

There are also certain situations that are overruled by a minimum age and these can be for those who are married, serving in a defence role or who have assets above a legislated threshold.

You need to be mentally competent

This is where a witness proves their value, as they should be able to attest that you were of sound mind when you wrote the will and when they witnessed your signing of it. If you have doubts that your will may be contested further, it may be appropriate to have a doctor's certificate included stating your mental disposition at the time of signing.

Needless to say, your witness also needs to be of sound mind and should they fail to be in the future it's recommended that you rewrite your will and have it witnessed by another person.

The will must be signed by the testator plus 2 witnesses

Each witness should observe the testator's signature plus each other's signature when signing a legal will. Some jurisdictions won't allow witnesses to be beneficiaries and therefore will annul some, or all, of a benefit left for them. Therefore, witnesses should be people who have no vested interest in the document.

Be careful who you try to disinherit

Most countries will overrule a legal will that disinherits a spouse unless provisions were made prior to the marriage that the spouse would not be a benefactor. Even then, the courts could still frown upon a pre-nuptial agreement if it can be proved that the testator was not completely honest in their full disclosure of personal assets.

Some states and territories will also disallow children below a certain age limit being disinherited by the testator. This is to ensure that minors are provided for after they have left with a guardian.

Appoint a guardian for your children

If you have young children that will need to be cared for once you pass on then a competent guardian will need to be appointed to oversee their well-being. It's appropriate to have discussed this with the possible guardian prior to the reading of your last will and testament so that they don't find themselves in an awkward position.

In some cases, an older child may be able to care and provide for younger siblings.

Anyone can be a beneficiary

While there are laws that ensure your estate will provide for your spouse and children, you may also choose anyone else to be a beneficiary of your assets. This could be specific people, trust funds, charitable organisations or the homeless guy who sleeps in your neighbourhood park.

The courts are more interested in ensuring that state or federal laws aren't contravened and that those who should receive a benefit will. The more complex you make your last will and testament the more likely it is to be contested and your wishes will be overruled.