Play sixteenth notes on the hi-hat 1 e + a 2 e + a 3 e + a 4 e + a…
You are now going to strike the snare drum on beat 2 and beat 4 by dropping your right hand down from the hi-hat instead of playing the hi-hat on those beats..
Practice at a slow tempo at first making sure your right hand flows smoothly between hi-hat and snare drum and that you strike the centre of the drum whilst all the time keeping a consistent tempo.