If you are using pip, you can simply run
pip install symfit
from your terminal. If you are using linux and do not use pip, you can download the source from https://github.com/tBuLi/symfit and install manually.
Are you not on linux and you do not use pip? That’s your own mess.
To also install the dependencies of 3rd party contrib modules such as interactive guesses, install symfit using:
pip install symfit[contrib]
pip install sympy pip install numpy pip install scipy