I would imagine that, like any bequest in a will, the main requirements will be...
- That there is no reasonable doubt that the request was made by you.
- That the request is phrased clearly (including if a lawyer has to pick it over!)
- That you are "sound of mind" enough to understand the consequences of what you are requesting.
I don't know that any of the above are specific legal requirements for a will (it will depend on the laws where you live), but having the above confirmed by an independent "responsible person" will make it much harder for your will to be challenged after your demise (for example, if surviving family members have different religious beliefs than you which require a consecrated burial.) A lawyer specialising in the field would be the ideal person to approach, but of course that could be expensive. However, a bit of research into your local laws might find a reasonable alternative for who could "witness" a will in order that your wishes will be respected.
Also, the health services where you live might offer a way to note your wishes by registering or carrying a card. For example, here in the UK, the health service promotes keeping a standardised "donor card" on your person so that, if you die in hospital, doctors can quickly establish whether your organs are available while they are still fresh enough to use for transplants.
_________________
When you are fighting an invisible monster, first throw a bucket of paint over it.