Spinoza is not in any straightforward way a follower of Descartes. Descartes, for instance, believes that there are two substances, mind and body. For Spinoza, by contrast, there is only one substance; thought (mind) and extension (body) are attributes of it. That's quite a contrasting picture ! It is certainly nothing like Cartesian dualism.
Freedom for Spinoza is not exemption from causality. We are free when we understand - have 'adequate ideas' about - the causes of our actions. To be free is not to be able to do simply what we choose but to understand the nature of the desires and emotions which cause our actions. It is understanding the internal necessity for our actions - recognising why, given the nature of reality, we act as we do. We no longer see our own nature as an external constraint; and realise that there is no freedom except to act in accordance with what we really are. When we have self-knowledge we are free since we understand our actions as the inherent and inevitable outcomes of our essential natures.
Few people have self-knowledge though it is in principle available to all. It is a product of reason.
One caution : like Hume later, Spinoza opposes freedom not to causation but to constraint. I am free if I act causally according to an essential nature which I understand. If you prevent me from acting according to that nature, say by coercing me, then Spinoza recognises that in this respect I do not act freely.
All this is set out in Ethics, I, Definition 7; I, Corollary 17 & Lemma 58; IV, Demonstration 67. Spinoza's 'Ethics; is a hard book to get to grips with : Stuart Hampshire's 'Spinoza' is an old book but still a lucid and broadly reliable guide.