• Make sure you have Java JDK 1.8+ installed
• Downloading it as zip or clone with Git
• Open console / Command Prompt in downloaded folder's location
• Run "gradlew build"

repositories {
	maven { url "https://maven.latmod.com/" }
}

dependencies {
	deobfCompile "com.feed_the_beast.mods:FTBLib:${ftblib_version}"
}

And ftblib_version=5.+ line in your gradle.properties file. You can also specify exact version, e.g. 5.0.0.

You probably also want to add "required-after:ftblib" or "after:ftblib" in your @Mod's dependencies. Most of the APIs don't need hard dependency, but things like GUIs won't work without FTBLib loaded