How do we practise a hard sound (e.g., /s/, /r/, "th")?

Little and often — short, daily, playful. Start where the sound is easiest (often the beginning of a word, next to a vowel your child likes), use a mirror and visual/touch cues, then move quickly from the sound alone → words → short phrases your child actually *wants* to say. Don't drill a sound in isolation for long. An SLP can pick the highest-impact sounds first rather than working through them in order.