Let's start by adding UxUi and PacketEvents to your project.

Gradle groovy DSL

repositories {
    maven {'https://repo.papermc.io/repository/maven-public/'}
    maven {'https://repo.codemc.io/repository/maven-releases/'}
    maven { url 'TODO' }
}

dependencies {  
    compileOnly 'io.papermc.paper:paper-api:VERSION'
    compileOnly 'com.github.retrooper:packetevents-spigot:VERSION'
    implementation 'TODO'
}

or kotlin DSL

repositories {
    maven("https://repo.papermc.io/repository/maven-public/")
    maven("https://repo.codemc.io/repository/maven-releases/")
    maven ("TODO")
}

dependencies {  
    compileOnly ("io.papermc.paper:paper-api:VERSION")
    compileOnly ("com.github.retrooper:packetevents-spigot:VERSION")
    implementation ("TODO")
}

Also use compileOnly if you have included it as a plugin.

How to init with Kotlin?

For kotlin projects:

    override fun onLoad() {
        PacketEvents.setAPI(SpigotPacketEventsBuilder.build(this)) //Set api for packetevents
    }

    override fun onEnable() {
        PacketUxUiAPI.init()
        val menuService = PacketUxUiAPI.getService()
    }

Or if you wanna init packetevents by yourself:

    override fun onLoad() {
        PacketEvents.setAPI(SpigotPacketEventsBuilder.build(this)) //Set api for packetevents
        PacketEvents.getAPI().load
    }
    override fun onEnable() {
        PacketEvents.getAPI().init
        PacketUxUiAPI.init()
        val menuService = PacketUxUiAPI.getService()
    }

menuService is a kind of manager for working with menus, opening them to players, etc.