These guidelines are current as of Archicad 19.
You can install Archicad from the disk image on the server. It's also available from Graphisoft directly. We keep copies of Archicad installers going back, well, back far enough that not all of them will run on our current platforms. They are located at 3 Resources/AC/AC Program Files/[versionNumber].
(A word on disk images because they are still weird after all these years. A disk image (extension: .dmg, often called a dmg) is file which, when opened, mounts a volume in the local filesystem just as if you had inserted a physical disk. It will appear in the Finder sidebar, and will probably open a window. Some applications are drag-installable, and to install the application you just drag it from that window to your Applications folder. Sometimes they even provide an alias to your Applications folder to make the process clearer. Once you have installed the application, you can/should eject the imaginary disk.)
Archicad is not drag-installable, so you need to run the installer. When the window opens you will see an icon named "Archicad 19 Installer". Don't run that. After a blissful two version hiatus, the extraneous flashy installer interface is back. Scroll down and open the "Archicad 19" folder. Double-click the "Archicad Installer" in there.
Archicad doesn't use the standard package installer. It uses a custom installer written in Java. Mac OS X currently ships without Java installed. Even so, you might/probably have Java installed because it's required for other things. If not, the system will prompt you to install it, which is pretty automatic and takes a couple minutes. Then the installer will launch. I ignore the warning about closing all other programs.
Follow the directions. AC19 uses only CodeMeter keys, so that option is gone. I don't customize the installation. Use the default location, /Applications/Graphisoft. I don't import Work Environment settings; as far as I can tell this has never worked properly. (Maybe it works now. Fool me 4-6 times, shame on me.) Yes on the Dock icon, no on the Desktop icon, yes on checking for updates.
New version installations usually require a restart, but in 19 it hasn't been the case for me. Restart if it asks you, but you need not restart immediately. If you need to apply hotfixes to the version you are installing, you can do so right away without restarting first or even running the original version. Run the main installer, run the most recent hotfix, then restart just once.
After restart, there are a couple things that are beyond the scope of the installer.
Goodies
Goodies, which term is just awful, are quasi-non-supported add-ons available from Graphisoft. Even though they don't support them. Three of them are core functions which for some reason Graphisoft likes to pretend aren't core: Accessories, Check Duplicates, and Polygon Count.
The goodies used to be separate downloads, but now they share one installer (at least in the USA version), where you can select which ones to install. The goodies installer is a dmg located on the server next to main Archicad installer. Run it and choose the goodies name-checked above.
Work Environment
Our Work Environment is heavily modified, and to me the default Archicad environment is profoundly disorienting and I can't do anything with it at all. So right after the first launch I import the work environment. The work environment is stored in the preferences, so if you are re-installing a version you have used in the past, the work environment might be OK at first launch.
DXF-DWG Translators
In version 19 I have abandoned custom translators. The default translators meet our needs, and if they need to be tweaked we will tweak them. The templates should default to "03 For as is output" and "02 For editable import".
DisableCrossPlatformMountingFeatures
If you ever take solo projects away from your usual network, and have external resources (libraries, modules, drawings) on a server, you need to know about dcpmf, to (help?) prevent Archicad from spending ten minutes wailing into the abyss about the mysteriously vanished IP address ZOMG.
Archicad Updates
Since you wisely opted to let Archicad check for updates during installation, it will do just that periodically (weekly, or change it in Work Environment), and eventually you will be notified upon launching that an update is available. It could be for the application or the Archicad library or both. Go ahead and do it; they usually fix more things than they break. The updates, identified by a sequential number like 6 and a build number like 5019, will wind up in your downloads folder. The files are disk images once again, and the update process is very similar to the original installation. It knows what to update and usually doesn't require a restart.
If you are installing a version that has an update, always install the goodies before the update.
If you are installing a version that has had several updates, you only need to install the most recent one; it includes all the previous fixes. We keep the most recent update dmg on the server next to the main installer.
The only caveat for updating immediately is if you are working in Teamwork. In that case, all users in a project have to update together, along with the BIM Server application which is running the project.