How to Build A Strumming Pattern
Now that you have the fingering for your first chord, let's start building the strum pattern with a bass note. The bass note you're going to use for the Em chord is the 6th string open. Alternate between playing the 6th string and strumming. For this exercise, just play downstrokes. The rhythm should be simple, relaxed, and metronomic.
Play along with the tab below a few times to get the feel of this pattern. The tab will loop automatically and you can play around with the tempo slider to practice at different speeds. At this point, slower is better to give yourself time to think about and internalize what is going on but once you feel comfortable at the slow pace, bump it up a few clicks.