1) Search support.steampowered.com for "server system requirements" or other like terms
2) Search support.steampowered.com for "firewall ports" or other like terms. Once you've found what ports are needed to be open, read your router/firewall manual. There are too many out to have just one answer for this.
3) Read the instruction with our ULX_readme.txt, that comes with the download package. (I'm sorry, this may seem short, but we hear this question too many times from
people that haven't read/followed the instruction, or at the least attempted to)