Start with four beats
Count 1–2–3–4 out loud and tap your foot. A downstroke moves towards the floor; an upstroke comes back up. Begin with four downstrokes right on the beats.
Count: 1 2 3 4 Strum: D D D D
Add upstrokes
Count 1-and-2-and-3-and-4-and. Keep your hand moving down on numbers and up on ‘and’, even if you miss the strings on a movement. Try down on 1 and 2, up on the and after 2, down on 3 and 4, up on the and after 4. Start very slowly.
Count: 1 & 2 & 3 & 4 & Strum: D D U D D U
Use a chord
Mute the strings with your fretting hand and practise the pattern first. Then play it on Em and later Am. Slow down whenever the beat becomes uneven. Consistency matters more than speed.