Wow, that was an awful suggestion.
Always, always, ALWAYS! make sure you can trust the deb you've downloaded.
Before you install a .deb
Before telling you how to install it, I would ask you to check if they have the software in:
flathub because then you can click install there, download the .ref, click it in files, and appcenter will offer to install it for you (which will add flathub as a source in appcenter, allowing you to search their apps through it). flatpaks run in confinement, and this adds a layer of security as they can't access all parts of your system (well, in theory)
Now, elementary OS does not have direct snap support, in that,
snapd is not installed by default. But this is another 'containerized' format, snapd keeps these apps up to date, which can either be annoying or practical depending on how you look at it.
If you decide to use snaps, I would suggest doing the following:
you can read instructions to get started with snapd here
install the third party snap frontend for elementary OS developed by Bart Zaalberg available here
Go to the snap store and install apps by clicking the install button in the right upper corner, and then clicking 'desktop store' then your URL handler should ask you how you want to open the URL for which you choose snaptastic.
Your options for installing a .deb the GUI way
So far as i am aware, you have two options for installing .debs through the GUI
Well, to install a .deb, i recommend installing Eddy it's a curated app, so it looks native, and lives in the AppCenter. The only thing I would note here, is that I have had a situation where it did not install a dependency.
If you want to have more details available whilst installing, you might go for gdebi instead, which you can also find in AppCenter. it is not a native app though and can look a bit weird.
Lastly the CLI option
If you decide to go the CLI route instead you can right click in your folder where the package (lets call it: blah.deb) is reciding, and click "open in terminal" and then type the following command:
sudo dpkg -i blah.deb
this may result in a partial install, say, if there are uninstalled dependencies, so you might want to follow it up with:
sudo dpkg --configure -a