If you are using the standalone dedicated server with a MariaDB setup then it's pretty straightforward. Anything else is complicated and I'm not going to try to explain for that.
As for setup using a standalone instance of MariaDB the simplest way would be to put this SQL code into the patch.sql file at the end and restart the server. An alternative is to use the included utility, HeidiSQL, as this will provide a GUI interface to the SQL data.
If using the patch.sql method you can verify by two methods. First, by testing character creation. If the SQL is setup correctly you will get an error on the character creation page as soon as you hit the Create/Done button at the bottom. Second, it can be verified by opening something like HeidiSQL that is connected to the SQL server and open up the lif_1 database. Scroll down and you should see an entry named 't_characterLimit' in with the other SQL tables and procedures.
If you don't want to restart the server or are nervous about putting it in patch.sql then you can use the HeidiSQL GUI to create the trigger. For this you need to load up the lif_1 database. Once it's open right click on any table/proc entry and select Create new -> Trigger. From there fill in the options details so that the CREATE code tab matches the raw SQL code of the mod. Hit save when everything is entered and that's it.
If you don't have knowledge of SQL DB administration then the first method using patch.sql and a server restart is going to be the way to go. Hope this helps