Don't forget to run the nearcore node first.
Run backend:
virtualenv .env --python=python3
# Activate .env, e.g. .env/bin/activate
cd block-explorer-api/
python setup.py install
cd src/near/block_explorer_api
python app.py
Run frontend:
cd web-ui/
npm install
cat "REACT_APP_API_URL=http://127.0.0.1:5000" > .env
npm start