DeclaraCAD is unstable so it's recommend to install from source as it's easier to update.
As of March 2021 DeclaraCAD has moved to a private repo and updates are no longer being published. Old forks are still floating around on Github if you want to try those. Sorry for the inconvenience.
Installing from source
To install DeclaraCAD and it's dependencies from source, git, pip, and conda are required.
# Install conda or miniconda
# See https://conda.io/miniconda.html
# Create a conda env
conda create -n declaracad
# Activate it (on windows just do `activate declaracad`)
source activate declaracad
# Install it OCCT and dependencies
conda install -c trelau -c conda-forge pyocct pysmesh
# Clone the repo
git clone .../declaracad.git
# Go inside the cloned repo
# Install declaracad
pip install -e .
declaracad to start it.