I love these kinds of pics xD so many great faces here! and lol RD bottom right xD Top right Rarity and mid right AJ are my favs xD rest are really cute like that bottom right Rarity and bottom mid Twi and top right Rainbow Dash ^^ all of it is a lot of fun!
Let me quote something generic. "Practice practice practice"
Learning the basics of drawing itself helps. Knowing to break down images into simple shapes, construction lines, and the like. As for facial expressions, look in the mirror, make a face, then exaggerate them in drawing. It also helps to watch cartoons/anime, sure watch for the story first, then go back,look at their facial expressions at given moments, it helps a lot.