Four beats, one chord
Count 1-2-3-4 aloud and strum Em on each beat. Keep your hand moving evenly. You can use a slow metronome and aim to play with the click, not after it.
Split each beat in two
Count 1-and-2-and-3-and-4-and. Move your hand down on numbers and up on ‘and’. First sound only the downstrokes; then add one upstroke without changing speed.
What if I lose the beat?
Pause and clap along first. Return to the guitar once you can feel four even beats. A slower tempo helps.