It is advised to use virtual environment to install the files. Virtual environment can be created using anaconda. See this to install conda into your system.
Once you have conda in your system you can use it to create virtual environment.

conda create -n name_of_the_env python=3

You can activate/deactivate the virtual enviroment

conda activate name_of_the_env
conda deactivate

Once you have activated your virtual environment go to the folder where you kept the cloned files.
To install the folder type in the terminal

pip install .

Once installed, files can be changed and run in python. As an example,

python

>>> from qiskit import QuantumCircuit,BasicAer,execute
>>> qc = QuantumCircuit(2)
>>> # Gates
>>> qc.x(1)
>>> qc.cx(0,1)
>>> # execution
>>> backend = BasicAer.get_backend('dm_simulator')
>>> run = execute(qc,backend)
>>> result = run.result()
>>> print(result['results'][0]['data']['densitymatrix'])

It would output the resultant densitymatrix as,

[[0 0 0 0]
[0 1 0 0]
[0 0 0 0]
[0 0 0 0]]

The jupyter notebook provide detail examples about how to use this simulator.