Design a Deliberate Practice Drill for Any Skill

Repeating what you can already do feels productive and changes nothing. Name the skill, your level and your target, and this designs a deliberate-practice drill that isolates the single hardest sub-component, pushes you just past comfortable, and gives you a success criterion you can judge on the spot.

I want to apply deliberate practice principles to improve at {SKILL}. My current level is {CURRENT_LEVEL} and my target performance benchmark is {TARGET_BENCHMARK}.

Design a focused drill session that isolates the single hardest sub-component of this skill. The drill should:
1. Push me just outside my comfort zone (not overwhelmed, not coasting)
2. Have a clear, measurable success criterion I can evaluate in real time
3. Last no more than 20-30 minutes of intense focus
4. Include a specific variation to run if I find it too easy or too hard

Also tell me what cognitive or physical quality this drill is actually training, and why that quality is the bottleneck at my level.