The safest way to use it is to download the code from the GitHub repo and extract it somewhere on your computer. Open the index.html file with your safe browser and you are ready to use it. Also, for an extra safety measure disable your internet connection and remember to always use a safe browser to do this.
I really hope you find this useful and it motivates you to learn more about NEM and blockchain technology in general. For any issues or improvement ideas or questions in general, don’t hesitate to drop me a message.