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Once you're able to produce a good sounding chord, try strumming it at different tempos and rhythms. Rhythm is based on your strumming pattern, and how long you're holding your notes. Try a basic 1-2-3-4 beat, otherwise known as a 4/4. The number on the top represents how many beats there are in the measure. Try strumming up and down in different progressions to create a different sound for your rhythm. Once you get a basic rhythm down, you can start to incorporate quicker or slower strumming. As you play your fingers might get sore. If this is the case, take a break and come back to practicing later.
Try out different strumming patterns and rhythms.