Eighth notes are two strums per beat: down on the number, up on the "and". Count "1 and 2 and 3 and 4 and".
The key idea is that your strumming arm never stops. It moves down on every number and up on every "and", like a pendulum, whether or not you hit the strings. Patterns are made by choosing which strokes connect.
Exercise one strums every stroke on Em and Am. Upstrokes only need the top three or four strings, so they're naturally lighter.
Exercise two is the pattern "down, down-up, up-down-up", in G major with G-Em-C-D. On each empty column in the tab your hand still moves but misses the strings, and the audio leaves a short silence: ease the fretting pressure so the chord stops, which gives the pattern a crisp feel.
Exercise three, "down, down-up, down, down-up", moves through Am-Em-C-G. The missed strokes are the upstrokes after beats 1 and 3.
The most common mistake is stopping the arm on the missed strokes, which throws the timing off. Keep the pendulum going, and say the pattern aloud while you play.
