My UG was also Psychology. The only "hard science" courses I took were intro chem, bio, physics, 3 years before I wrote the MCAT. My psych major definitely helped for the P/S section of the MCAT, as well as CARS. Because I had no background in orgo/biochem/genetics, I knew I would have to put in extra days to self-teach/practice those topics.
I studied Jun-Sept, using ExamKrackers books, KhanAcademy passages, the AAMC official materials, and YouTube. I was lucky to not have to work during this time but I know that's not possible for everyone. I probably could/should have studied in less time, but I gave myself a lot of break days so I wouldn't burn out.
Lmk if you have any specific questions about the process or would like to see my schedule.