A Will, just like any other legal document must be made in front of a Notary.
So why waste any money on a solicitor.
Go to a Notary. Take your ORIGINAL passport as ID.
Tell them what you want. They will draw it up and you can sign it.
You will get a certified copy.
The Will is then registered by the Notary at the central registry in Madrid.
When the time comes, any solicitor can search, find where it is recorded, and access it.