Start with one motion
Hold a pick lightly or use the side of your thumb. Brush downward with a relaxed wrist. For Em you can strum all six strings; for Am start on the A string and avoid the low E.
Add upward strokes
Count 1, 2, 3, 4 aloud and strum downward on each beat. Then add gentle upstrokes between beats: 1-and-2-and-3-and-4-and. Let your hand keep moving even when you skip a stroke.
How hard should I strum?
Only hard enough for the intended strings to sound clear. A tense, forceful stroke usually makes timing less even.