Learn three more sounds: F (fff), L (lll), and W (wuh), noticing how differently your mouth moves for each.
F says fff, blowing air gently through your teeth. L says lll, tapping your tongue behind your top teeth. W says wuh, with round, puckered lips.
Fff, lll, wuh -- three more sounds for your growing collection.
Mastery looks like: They can produce the F, L, and W sounds with roughly the right mouth shape and sort familiar words among them.
Common stumbles: Swapping the lll tongue-tap for a wuh lip-round, since both can feel similar to a beginner; rushing through the air sound fff.