First create a virtual environment:
virtualenv ~/venv/nnUNet -p python3

Activate the virtual environment:
. ~/venv/nnUNet/activate

Install all the required packages - they are written in requirments.txt, found here :
pip install ~/software/src/venv/nnUNet/requirements.txt

In order for all the packages to match, we have to install these separately:
pip3 install torch torchvision torchaudio --extra-index-url https://download.pytorch.org/whl/cu113

Than also install nnUNet:
pip install nnunet

And you are done! You have created nnUNet virutal environment!

To deactivate virtual environment: deactivate