The phrase "in my free time" is correct and usable in written English. It is used to describe what activities you do when you are not working or studying, e.g. "In my free time I like to read books and go for walks in the park.".
"I do it in my free time.
In my free time, I had shows.
In my free time, I enjoy playing and writing games.
In my free time: I play basketball every Sunday.
"In my free time I play the guitar".
"In my free time, I play some piano.
I'm a neuroscientist who does volunteer work with the elderly in my free time.
Thanks to Ludwig my first paper got accepted! The editor wrote me that my manuscript was well-written
Listya Utami K.
PhD Student in Biology, Bandung Institute of Technology, Indonesia