FabricLoader 0.15.0+ includes MixinExtras already. If you want to maintain compatibility with older versions, or want to use a different version than is provided, see below:

dependencies {
    include(implementation(annotationProcessor("io.github.llamalad7:mixinextras-fabric:0.2.2")))
}

NeoForge 20.2.84+ includes MixinExtras already. If you want to maintain compatibility with older versions, or want to use a different version than is provided, see below:

dependencies {
    implementation(jarJar("io.github.llamalad7:mixinextras-neoforge:0.3.5")) {
        jarJar.ranged(it, "[0.3.5,)")
    }
}

dependencies {
    compileOnly(annotationProcessor("io.github.llamalad7:mixinextras-common:0.3.5"))
    implementation(jarJar("io.github.llamalad7:mixinextras-forge:0.3.5")) {
        jarJar.ranged(it, "[0.3.5,)")
    }
}

dependencies {
    compileOnly(annotationProcessor("io.github.llamalad7:mixinextras-common:0.3.5"))
    implementation(include("io.github.llamalad7:mixinextras-forge:0.3.5"))
}

This is only a rough guide. You will need to look into the specifics of setting up ShadowJar for your platform.

plugins {
    id "com.github.johnrengelman.shadow" version "8.1.0"
}

configurations {
    implementation.extendsFrom shadow
}

dependencies {
    shadow(annotationProcessor("io.github.llamalad7:mixinextras-common:0.3.5"))
}

shadowJar {
    configurations = [project.configurations.shadow]
    relocate("com.llamalad7.mixinextras", "your.package.goes.here.mixinextras")
    mergeServiceFiles() // Very important!
}

To initialize MixinExtras, simply call

MixinExtrasBootstrap.init();

somewhere suitably early. In almost all cases I would recommend making an IMixinConfigPlugin and initializing MixinExtras in its onLoad method.