On the way up, make sure your wrist doesn’t go higher than your “ceiling” which could be any visual marker you choose. (Maybe the top curve of the body of your guitar, etc).

On the way down, make sure your wrist doesn’t go lower than your “floor” which could be maybe the lower curve of the body of your guitar.

This will at the very least give you a range of motion that you shouldn’t exceed when strumming, which will then improve your consistency.

Much Respect,
The Musician’s Toolbox

In music notation, you would see this as a rest or a tie, but you don’t have to know all that just yet, just try this little exercise:

Strum down and up on your guitar strings as if to a ticking clock. Tick (down) Tock (up).

Keep this going, down, up, down up, evenly, like a clock. Congratulations, you’re strumming. Ok, now try this pattern:

Tick Tock Tick Tock Tick Tock Tick Tock
Down Miss(up) Down Up Miss(down) Up Down Up

It will help to memorize it, but as you get it into your muscle memory (so you’re not thinking about it as hard), gradually speed up. Remember a miss is where you purposefully miss the strings. You still make the motion, but you just miss instead.

Perfect Practice Makes Perfect.

Much respect,
The Musician’s Toolbox