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I take notes on absolutely everything, but they turn out to be useless later.
Literal
Anything-and-everything memo [object-を] do [but-が] later useful-thing become-not.
A relatable self-deprecating remark. 何でもかんでも is a reduplicative emphatic phrase meaning 'anything and everything,' 'every single thing,' often with a faintly dismissive or indiscriminate edge — the speaker is admitting they don't filter what they write down. 使い物になる is a fixed expression meaning 'to be useful,' 'to be of practical use'; the negative 使い物にならない ('useless,' 'no good for anything practical') is just as common, often applied to people, tools, or in this case notes. The が at the seam is the contrastive 'but,' setting the noting-it-all action against its lack of payoff.