How to use Musical.ly, the app with 70 million users that teens are obsessed with

iStock.com I remember the days of coming home from school and tuning into MTV's "TRL," a program that counted down the top music videos in front of screaming mobs of adolescent girls. Today, teens are finding the same guilty pleasure in an app. Musical.ly is a DIY music-video making app, where users (or "musers," as they're called) create 15-second videos of themselves lip-syncing and dancing along to popular music. You can "heart" videos and create digital duets with your "BFFs," or people yo...