In general, learning to learn and be patient with the process. Based on your specific context, I’d also recommend reading code, a lot of it. There’s definitely a difference between designing and writing software in isolation and jumping into a new code base. Learn about what it is supposed to do and why, then spend a lot of time reading the code to learn the how. Also, beware of being overly critical without the history and context behind things—but, learn from the mistakes made previously and raise the quality bar by example.