Add this repository to your pom.xml

<repository>
    <id>codemc-releases</id>
    <url>https://repo.codemc.io/repository/maven-releases/</url>
</repository>

Add the packetevents dependency to your pom.xml.
Replace PLATFORM with the platform you are developing for. (Supported platforms: spigot, bungeecord or velocity)

<dependency>
    <groupId>com.github.retrooper</groupId>
    <artifactId>packetevents-PLATFORM</artifactId>
    <version>2.4.0</version>
    <scope>compile</scope>
</dependency>

Add this repository to your build.gradle

maven { url 'https://repo.codemc.io/repository/maven-releases/' }

Add the packetevents dependency to your build.gradle
Replace PLATFORM with the platform you are developing for. (Supported platforms: spigot, bungeecord or velocity)

implementation 'com.github.retrooper:packetevents-PLATFORM:2.4.0'

Now that you have the dependency, you can use the maven-shade-plugin(on Maven) or shadowJar(on Gradle) to shade this dependency. Please relocate the following packetevents packages to avoid issues with multiple projects shading the library. Pick unique packages.

com.github.retrooper.packetevents -> me.username.firstplugin.packetevents.api
io.github.retrooper.packetevents -> me.username.firstplugin.packetevents.impl

The relocated packages I made up are just examples, you may pick whatever you wish.

You can minimize the dependency to save space. Classes you don't use will not be shipped with your plugin allowing you to have a much smaller jar. This should clear up the packet wrappers you aren't using. On Gradle you insert a minimize() inside the shadowJar task. On Maven you can set <minimizeJar> to true in your maven-shade-plugin configuration.

If you are developing a Spigot/Paper/etc server plugin, read this carefully. You are going to want to open up your plugin.yml and soft-depend these plugins:

softdepend:
  - ProtocolLib
  - ProtocolSupport
  - ViaVersion
  - ViaBackwards
  - ViaRewind
  - Geyser-Spigot

This ensures that our injector is loaded last and will therefore operate correctly.

If you need any more support, consider joining the discord server: https://discord.gg/prqzNg9tRU