How do you actually make a will, and what makes one invalid?
A will must satisfy specific formal requirements, and failing any of them makes it invalid regardless of how clear your intentions were — which is why a large proportion of the difficulty around wills concerns execution rather than content.
What a valid will requires in England and Wales:
In writing.
Signed by you, or by someone else in your presence and at your direction.
Signed in the presence of two witnesses, present at the same time.
Each witness then signing in your presence.
Capacity — understanding the nature of the act, the extent of your property, and who might reasonably expect to benefit.
Intention, and freedom from undue influence.
The mistake that destroys gifts. A witness — or their spouse or civil partner — cannot benefit under the will. The will remains valid; the gift to them fails entirely. This is the single most common serious error in home-made wills, and it is invisible until death.
Other common failures: unclear wording creating ambiguity; failing to appoint executors or appointing someone unwilling; not covering what happens if a beneficiary dies first; gifting something already sold; and marriage, which automatically revokes an earlier will in England and Wales unless made in contemplation of that marriage — a rule that surprises almost everyone.
Divorce does not revoke a will, but treats a former spouse as having died, which can produce unintended results.
What a will should cover: executors; guardians for children under 18, which is frequently the most important clause and is available nowhere else; specific gifts; the residue — everything not specifically gifted, which is where most of the estate usually sits; substitute beneficiaries; funeral wishes, which are not binding; and any trusts.
What a will does not control: jointly held property passing by survivorship; pensions and life policies paid at a scheme's discretion or in trust — which is why beneficiary nominations matter and are separate; and assets already given away.
Practical points: store it safely and tell your executors where; do not attach anything to it with a staple or paperclip, which raises questions about removed documents; review after marriage, divorce, births, deaths and property changes; and amend by codicil or by making a new will rather than by writing on it, which has no effect.
General information, not legal advice.