How do you apply SOLID in a legacy codebase?

Applying SOLID to a legacy codebase is an incremental, risk-aware process called the Boy Scout Rule approach (leave the code a little better than you found it). The strategy: (1) Write tests first — before refactoring, ensure characterization tests that capture current behavior; (2) Extract methods — break long methods into smaller, named functions (SRP at method level); (3) Extract classes — identify distinct responsibilities in God Classes and extract them; (4) Introduce interfaces — wrap external dependencies (DB, email) in interfaces and inject them (DIP); (5) Use the Strangler Fig pattern — gradually replace old code paths with new, SOLID-compliant code rather than big-bang rewrites. Never apply SOLID to untested legacy code recklessly; each refactoring step must be backed by tests.