First, if you haven't done so already, download the torrent (Updated 6-15-2019) for the setup installation files. You will need a torrent client to download the torrent. Popular clients include Deluge and qBittorrent.
For this guide, we'll be using Deluge.
Once you've installed a torrent client, you will then need to open the torrent inside of the torrent client itself. For Deluge, we can add the torrent by dragging and dropping the torrent into Deluge itself. For other clients, usually double-clicking the torrent file will do this, but if it does not, then you could click on File, then selecting "open/add torrent" in most torrent clients to do the same.
Upon opening and adding the torrent in your client, it will then begin the download process. Do note, that the speed of the torrent depends on your download speed to the server and how many seeders (people who are at- 100%).
Once the torrent has reached 100%, it will start to seed. This means it is done and now helping others download. It is now time to open it and run the installer.
In Deluge, right-click the torrent named "PSO2_5.0901.0" and click on "Open Folder". Other clients can allow you to open the torrent's download location by double-clicking. Inside, you'll see a file named "PSO2_EP5_SETUP_8_31_2018.part01.exe" followed by a bunch of other rar files. You will want to open the exe file as administrator.
Upon running the installation setup, please read the agreement terms and click on "Agree" to proceed.
You will now proceed to the installation section. By clicking on "Browse", you can choose where you wish to install the game. Once you've selected where you want to install to, click on "Install".
This will now proceed to install the game. If you need to pause the installation, you can click on "Pause". This will halt the installation temporarily, but you cannot resume where you left off if you close the setup. Click on "Continue" to resume the setup, if you have paused it.
Once the installation has finished, the PSO2 Tweaker will launch.