As I understand it, the preposition de is used as either 'from', 'made from' (material), 'of' (possessive), or for time frames.
So why is 'de' used in this sentence?
Estoy totalmente de acuerdo.
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You shouldn't expect every expression in Spanish to be analyzable from the literal translation of its parts in English. De is a preposition that appears in many, many idioms and expressions, which sometimes only make sense as a whole.
De acuerdo means "in agreement". It's a fixed expression. The preposition de cannot be translated, and indeed, the opposite of this expression uses a different one: en desacuerdo. You can use it with the verb estar and you can conjugate the verb, negate it and/or add an adverb like totalmente, but that's about the extent of modification it tolerates.
(Beware, de acuerdo con... by itself also means "according to...".)
Prepositions are much harder to get to grips with if you try to translate them literally from one language to another. It we made your example plural we might say in English
We are in agreement
We are of the same mind
We are with you on that one
And probably some others I cannot think of just now. So we are not unanimous In English about what preposition works best here.