Port forward + making exception in eventual local firewall on the machine that's hosting, that should be it.
(if you're on windows - EDIT: And only use the built in one - you can do this: open "RUN" windows key + R - paste this:
powershell -command "start-process powershell -command "New-NetFirewallRule -DisplayName "Allow 25565 for Minecraft Server" -Direction Inbound -Action Allow -Profile Any -LocalPort 25565 -Protocol TCP" -verb runas"
and press OK.
Explanation of what it does:
Opens PowerShell as admin and adds a rule to allow incoming traffic on port 25565 and setting a name for the rule)
You can use this tool to check if the server is responding from the outside:
https://dinnerbone.com/minecraft/tools/status/
In server.properties - leave that empty