Separate the rhythm from the chord
Before playing, count “one, two, three, four” repeatedly at an even pace. Tap gently along with the count. The gap from four back to one should feel the same as the gaps between the other numbers.
You can use an external metronome at a comfortable speed, but the first task is simply to hear and repeat an even count. This page does not produce a metronome click.
Choose one familiar shape
Form a chord and check its included strings. Open E minor works well for this exercise because all six strings are included.
If forming the shape takes attention, make it before starting the count. There is no need to add chord changes while you are finding a steady strum.
Add a downstroke on every count
Make one gentle downward stroke on each number: down on one, down on two, down on three, down on four. Return your hand to its starting position between strokes without sounding an extra strum.
Keep the motion small and manageable. If the pick catches or the notes sound uneven, slow down and try a lighter stroke.
Practise stopping as well as starting
Play one group of four beats, then pause. Count another group without playing and begin again on one. This gives you a clear start and finish rather than an exercise that drifts until something goes wrong.
When one group feels comfortable, play two groups in a row. Listen particularly to the join between four and the next one.
Change one thing at a time
Once the four downstrokes are repeatable, you might practise a slightly different speed or make beat one a little more noticeable. Keep the chord the same while you explore that change.
When you later add another chord, reduce the speed if needed. A clean shape and a steady count are separate skills that become easier to combine through attentive repetition.