Nereus is an experimental Minecraft: Bedrock Edition server software written in Node.js.
Start by cloning this repository. If you don't have git just download the ZIP and extract it as "Nereus".
git clone https://github.com/ConflictPE/Nereus
Now install the required dependencies. You will need npm to do this.
cd Nereus && npm install
You should now be able to run Nereus from the command line.
node main