Having a professionally written will is definitely recommended, including the nomination of a competent person as executor. Preferably somebody who is familiar with Spain and knows how to go about approaching officialdom here. Definitely discuss this with a lawyer as there are things you can and can't do, and you wouldn't want a court to invalidate your will over a technicality.
Also bear in mind that if your heirs are not immediate family, they may be taxed on their inheritance, so make sure they understand the consequences. If they are relatives but its not obvious, make sure they have the paperwork to prove it.
If an escritura goes missing, it's not the end of the world as you can get a copy from (I think) the land registry. But it might save some effort if you ensure everybody who might need one gets a copy. It might also help to print out the pages covering your property from the catastro (you can access these online) so your relatives know exactly what it is they are inheriting, and maybe also attach the most recent valuation you have. I would recommend ensuring the main beneficiaries have a list of all your assets, that is names of banks, numbers of bank accounts, stock portfolios etc, so nothings gets forgotten. A hand written or self typed sheet is good enough. If you have a lawyer you trust, I would recommend putting that person on the sheet as well so they have somebody they can approach with confidence.