1. Build a functional test system, and write a big suite of tests which ensure preservation of behavior. Make it easy to run and see the results.

2. Slowly start extracting code and making small functions. Document like crazy in the code as you learn. Keep the single file or close to it, and don't worry about frameworks yet.

3. Introduce unit tests with each new function if you can.

After all that is done, make a plan for next steps (framework, practices, replace tech etc).

Along the way, take the jr backend engineer under your wing, explain everything, and ensure they are a strong ally.

Call me crazy, but that project sounds like fun.