Set the metronome for a slow pace. Try 40 or 50 beats per minute to start.
Mute a chord with your fret hand. To do this, place your fingers in position to play the chord, but do not press down on the strings.
With the beat of the metronome, strum down on the strings with the main beat, which will be each click of the metronome.
At the pause between beats, fret the notes on the chord, and hit the strings with an upstroke.
Increase the speed of the metronome as you get more comfortable with the playing.
Once you feel you can consistently hit the upstroke on the off beat, try removing the downstroke on the main beat. Count in your head, 1, 2, 3, 4, with each click of the metronome representing the 1 or the 3, and just hitting the upstroke on the 2 and the 4.