Good strumming starts with one motion done perfectly in time: a downstroke on every beat.
Hold the pick loosely and let the strum come from the wrist and forearm, like shaking water off your hand. Brush through the strings in one smooth movement; all the strings should sound together, not as a slow rake.
The first exercise is just E minor, four strums per bar. Count "1, 2, 3, 4" out loud. In the second, a G-C-D-G loop in G major, beat 4 is a rest. Stop the sound by lifting your fretting fingers off the strings while still touching them, and use that beat to move to the next chord. The third accents beats 2 and 4, marked > above the tab, where a drummer hits the snare.
It should sound like a steady march. The most common mistake is speeding up, especially on the beat after a chord change. The other is stopping your arm on the rest: keep the down motion going in the air so you stay in time.
