How do I stop making silly calculation mistakes in JEE Main even when I know the method?

Silly mistakes are usually a process problem, not a knowledge problem, so fix your process. First, slow down on the setup: write the given data and what is asked clearly, because most sign and substitution errors start from rushing the first line. Keep your rough work organised in columns instead of scattering it, so you can trace a slip quickly. In mocks, maintain an error log noting the type of each careless mistake, and after a few tests you will see your repeat patterns, like sign flips in integration or unit conversion in physics-style problems. Verify with quick sanity checks: estimate the order of magnitude, check limiting cases, and confirm the answer matches the units or expected range. Lastly, do not rush easy questions to save time for hard ones, because the easy marks are the cheapest.