Let’s take the installation of matplotlib as example.
Firstly, get the archive from the internet, you can find most of the archives from pypi.org.
$ wget https://files.pythonhosted.org/packages/26/04/8b381d5b166508cc258632b225adbafec49bbe69aa9a4fa1f1b461428313/matplotlib-3.0.3.tar.gz
Then uncompress the tar.gz file
$ tar -xvzf file.tar.gz
Go to the uncompressed file and install it
$ cd matplotlib-3.0.3
$ python3 setup.py install
Now you have installed the package manually.
The process is equivalent to the following:
$ pip3 install matplotlib
But with this method, you will be able to install some of the packages that are not supported by pip installation.