It's not hard, if you know what you're doing. But if you've never forwarded a port before, then it's a huge trip into unknown territory. And some, no, most people simply lack the knack for things like this. (There's no offense meant here; we all have our own unique skills. One is handy with computer networks, another can dismantle and put together car engines.)
Different modem/router brands all have their own configuration pages, some with really obscure and unclear settings pages for port forwarding.
Also, not really the case in my country, but some ISPs in some countries apparently don't allow you to change your modem configuration, so in that case you're forced to call their helpdesk.
This site might help:
Open Port
You'll need to find your modem/router brand and type in this list (and, if any, click the annoying ad away in the right corner after you've clicked on your modem).
The follow-up page should then help you out with screenshots and the like.
If you're already feeling overloaded by looking at that website's pages, I'd recommend not bothering and host games through Steam.
As an ex-IT support person, one thing I've learned is that computer configuration is not for everyone. You either have some natural knack for it, or you're hopelessly lost.