I take the view that "free will" is poorly defined, roughly echoing Strawson's compatibilist view (if you are interested in this you MUST read Strawson's 'Freedom and Resentment' [1962], one of the greatest philosophy lectures ever which changed the minds of many in the field; it's only 15 pages!). What matters to me is 'choice'. Now, seeing as I also hold a materialist view of the brain-mind issue, and a materialist view of metaphysics, I fully accept that all things we typically call 'choices' are causally (fully deterministically or partially randomly) determined by the material world, included both brain activity and everything else. We slide between different definitions of choice in everyday language, considering choices free-er when for example not taken under duress. But these definitions of choice are slid between precisely because we wish them to align with our understanding of ethics; specifically, what a moral choice is and when a choice should or shouldn't be punished. And this is all above board because even though there isn't some external presence disconnected from the material controlling the mind, nevertheless choices are being made within brains. Choices you make are yours because they stem from your brain's activity, and this activity IS you. Note that if you read Strawson's paper, he makes his arguments without appealing to materialism; it's just easier and quicker for me to use it because I believe it.