Start by finding what's untested. "Which functions in src/payments/ don't have test coverage?" Claude uses Glob to find the source files, then Grep to find the corresponding test files, and anything missing comes back in the results.
Then ask Claude to generate test scaffolding that matches your existing style. Point it at a test file you're happy with: "Generate tests for processPayment() following the same structure as @src/payments/validate.test.js". It matches your describe/it nesting, your mock patterns, and your assertion style. Add edge cases manually: empty inputs, null values, network timeouts. Then run the suite and fix failures before moving on.