1. Stop your server.
2. Click "Configuration Files"
3. Click "Config Editor" or "Text-Editor" next to ServerSettings.ini
4. Find the setting "EnableWhitelist" tick the box / change the value to True
5. Save in the top left.
6. Go back to "Configuration Files"
7. Click "Text-Editor" next to whitelist.txt
8. Add the SteamID64 of the player you want to whitelist, if you're adding multiple players, add them on seperate lines.
   
   For example:
   76561198023797809
   76561198113947883
   
   If you are unsure how to find a player's steamID64, get their profile URL and enter it at this site: https://steamid.io/
9. Save the file in the top left and startup your server, it is now whitelisted.