I find a lot of music with only chords:
They don't tell me the pattern I should strum. How can I know when to strum up or strum down?
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For this, you can either go by ear by listening to the recorded song, or by feel (i.e. how do you WANT to play it?). As far as up and down strumming, a general rule of thumb is that - regardless of the actual strumming pattern - you should maintain a steady up/down rhythm. Then you will simply hit the strings when you want sound and not hit them when you don't.
For most basic strumming patterns, consider down-strumming every quarter note and up-strumming on the eighth notes in-between.