Rhythm is about accuracy, keeping the spaces between each strum even. With the example below, play an E Major Chord and have a go at strumming the chord along with the counts. Use the 4 beat count in to help you with your timing.
Here is a repeating animation of what is written above, if you loose your place or go out of time when you're strumming, stop and wait until the 1st beat comes around again. Go over this until you are perfectly in time with the red strums.
Tip: Say this when you're playing it,
"Down Up Down Up Down Up Down Up"