In Spanish-speaking countries, you ask the person (or check how does he do it).
In our legal systems it’s not common to use initials; if we want to prove that we have checked and agree with every page of a contract we sign every page, not initialize it. We use initials in ways that are much less formal, such as marking school clothes, where the point is to be able to identify your item and who cares if you marked it xXx, or lists of who has to do which task within a team - in this case, if you have an Ignacio Ochoa and nobody else whose firstname begins with an I he’ll likely be marked just by I; if you have him and an Ismael Pérez, they’ll be IO and IP… or even O and P, if these can’t be mistaken as identifying somebody else. Most people in Spain will need an explanation of what the heck are you talking about, if you ask them what their initials are; in some other countries they’re more likely to have encountered the notion before.
When I’ve had to initial stuff in the US I’ve used MLO, which are the first three capitals of my full name. My full initials would be MdlLOdOV, and sometimes people have used MLODOV, MOOV or MODOV. The M, ML or MdllL are from my firstname (which is compound, not two or four names), the O or OdO from my compound first lastname.
del Carmen is NOT a middle name. Her full firstname is María del Carmen. It’s one of the advocations of Our Lady; the “del Carmen” part makes no sense without the María. Someone (specially a man) can be a Carmen without the María del, but you can’t be a del Carmen without the María.