Could someone please explain the sentence for me? And any citation of such usage?
You’ve heard about what you make of yourself when you assume things.
This is from the book Hacking for Dummies, 4th edition .
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In this context, the allusion is to a saying "when you assume things you make an ass out of (yo)u and me", which I recall hearing in The Silence of the Lambs and can be found in numerous places on the net.
Normally, to make something of yourself would be to achieve a notable or successful place in society, but that isn't what is intended here.