1. Four frets, four fingers
On one string play frets 1–2–3–4 using index, middle, ring and little finger. Pick each note once. Put your fingers just behind the frets, not on top of them, then change strings. Stop if your hand hurts.
2. Change between two chords
Play Em, release your fingers, form Am and strum it. Repeat slowly without a timer. Once the strings ring clearly, count four beats per chord and switch again.
3. Keep a steady pulse
Tap four beats with your foot. Strum down once on each beat, first over muted strings and then on Em. Work in short rounds and rest your hands; squeezing harder won't make your playing more accurate.