Connecting to the Minecraft Wizarding World
First, you need a Mojang account for Windows, macOS, or Linux. After you buy it, download and install the launcher.
If you want to try Minecraft out before you buy the game, check out the demo.
The first time you run the Minecraft launcher (and occasionally afterward if you don't play for a while), you'll have to log in using the Mojang account that you just created.
Once you've logged in, you can go right into the game by clicking on the big green PLAY button, but there are a couple other things you can do here, also.
The Skins tab allows you to upload a new skin (appearance, costume, etc.) for your character. There are a wide variety of Minecraft skin editors available online; The Best Minecraft Skin Creators article will help you pick the one best suited to your needs.
The Installations tab is where you will manage the different versions of Minecraft that you play. The Floo Network's Wizarding World that you are here to play requires version 1.13.2 of Minecraft, so you will need to use the Installations tab right now to install the correct version.
Click on the Installations tab and then click on ⊕ New….
Fill in a Name (it isn't important) and then select version release 1.13.2.
Optionally, but recommended, give this version as much memory as possible. Click on More Options and then edit JVM Arguments.
The first option (-Xmx2G) tells Minecraft that it can use a maximum (mx) of 2 gigabytes (2G) of memory.
Increase the value from 2 up to half or even three-quarters of the memory in your computer. You won't want to run other large applications at the same time as Minecraft, though. Then again, why would you?
Examples:
-Xmx4G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=32M
-Xmx8G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=32M
Whether or not you changed the JVM Arguments, click Create then Play to download and install Minecraft version 1.13.2.
As soon as the Minecraft main menu appears, click Quit Game, as there is one more necessary step to take.
Now you are going to install a graphics performance mod for Minecraft called OptiFine. Download it at https://optifine.net/adloadx?f=OptiFine_1.13.2_HD_U_F5.jar&x=0c2a.
Due to the different ways Java is installed on and/or supported by Windows and Mac, this is the messiest step. Hang in there…
After you have downloaded OptiFine, double-click on the OptiFine_1.13.2_HD_U_F5.jar file wherever your browser downloads files to.
If your Operating System gives an error that Java is not installed, download Java from https://www.java.com/en/, install it, then retry running the OptiFine installer.
If you are on Windows, it may/should give you some security warnings which you can confirm and proceed. I don't have a Windows system to test on.
If you are on a Mac, macOS will give you an error:
Open System Preferences and click on the Security & Privacy icon. Make sure the General tab is active. The window will look something like this:
Note the error message: “Optifine_1.13.2_HD_U_F5.jar” was blocked from opening because it is not from an identified developer.
Click Open Anyway. You will get another warning:
Click Open.
(Both Windows and Mac again) The OptiFine installer window looks like this:
Click Install. When the installer is done, a new window appears:
Click OK.
Congratulations! You're done with software installation. There are a few settings yet to change in Minecraft before you can play.
Run the Minecraft Launcher again. To the left of the large, green PLAY button is a pop-up list of versions of Minecraft. You should see OptiFine as an option now. Choose it, then click PLAY.
When you click PLAY, the Launcher will run the selected version of Minecraft. After the Mojang splash screen disappears, you'll see the Minecraft game options. Click Options.
Now click Resource Packs… and Open Resource Pack Folder….
You'll be taken out of full-screen Minecraft back to your desktop and a file explorer window will appear.
Download, by clicking the large green rectangular button in the upper right corner (not any of the distracting buttons on ads), Floo_Network_Alpha_0.5_-_Resource_Pack.zip (normal version) or Floo_Network_Alpha_0.5_-_Light_Resource_Pack.zip (simplified version for older hardware) and move it to the resourcepacks folder.
Close the resourcepacks folder and Alt-Tab (Windows) or Command-Tab (macOS) back to the game.
When you're back in Minecraft, the Resource Packs screen will look something like this:
(you may need to exit (Done) and reenter (Resource Packs…) for the lists to refresh)
Click on the right-pointing triangle ▸ to move Floo_Network_Alpha_0.5_-_Resource_Pack.zip from the Available column to the Selected column.
Click Done to return to the main options menu.
The last set of options to change allow OptiFine to have the right settings from the very beginning of game play. The game will require you to check these settings before entering the Wizarding World, but it's easiest to do it first.

Video Settings → Quality → Mipmap Levels: 3 → Done Do not try to drag the slider, instead click about 3/4 of the way along the bar until it is set to 3. Minecraft will freeze for a few seconds each time this setting is changed.

Video Settings → Render Distance: 8 Normal

Video Settings → Performance… → Smart Animations: ON

Video Settings → Performance… → Fast Render: OFF → Done

Video Settings → Details… → Alternate Blocks: ON → Done → Done

Chat Settings → Chat Background: OFF → Done

Language → Force Unicode Font: OFF → Done

Options → Controls → Reset Keys → Done → Done
Finally, create a Multiplayer server entry.

In order to connect to the Wizarding World, you'll need to click Add Server to create a menu entry with the correct settings.
In the Edit Server Info screen, type in a Server Name.
This can be anything you want—it doesn't have to be the words Wizarding World.
The Server Address has to be exactly mc.codecats.us to work correctly.
Finally, click the Server Resource Packs button until it reads Enabled.
After you have checked to make sure that everything is correct, click Done.
You will now see an entry for the Wizarding World in the list of multiplayer servers. You'll see the description you gave in the previous step, then The Wizarding World of Harry Potter, which the server sent to your game, and underneath that you may see an additional message from time to time, probably asking you to visit this web site to get more information.

Click the right-pointing triangle ▸ or double-click the new entry you made or click on the entry and then click Join Server
Troubleshooting:
If you do not see, on the right-hand side, the current and maximum number of players (“0/4” in the screenshot below) and a green signal strength chart, click the Refresh button. If it doesn't appear, double-check your settings. If it still doesn't appear, let Faolan know.
If you see, on the right-hand side, a red X, possibly with other information, then your version of the game does not match the version of the game running on the server. Let Faolan know.
Welcome!
Important note: if you are going to play with other players, you all have to enter at the same time. Late entrants will be treated as spectators by the game. I recommend using Mumble for audio chat to coordinate game play. This setup is new for me, too; we'll figure it out over time.