Download and install Node.js LTS
Download and install Git for Windows, and make sure the git executables are in your path
Download and install MongoDB for Windows, and make sure the mongo executables are in your path (redis for Windows is difficult to get working)
Follow the configuration instructions for MongoDB here (Yes, they're in the Ubuntu install docs, but don't worry. All of the same commands work in Windows)
Enter the directory which you want to install NodeBB in, and open a command prompt or powershell window there
git clone -b v1.8.x https://github.com/NodeBB/NodeBB.git
cd nodebb
./nodebb setup, and follow the instructions
Then you should be able to start NodeBB with ./nodebb start