On Linux/macOS:

$ cd project_dir
$ virtualenv -p python3 venv
$ source venv/bin/activate
$ ...
$ deactivate

On Windows:


The virtualenv command will create a copy of python in the given directory inside the current directory. In the above example it will create the copy in the 'venv' directory inside the 'project_dir'. After source-ing the 'activate' file the PATH will include the local python with a local version of pip. This requires bash or zsh.
See also the Python guide.