Stuffing them with fabric gives them a finished but soft look.
You will need:
scraps of t-shirts in the colors you want
hot glue gun
dremel or drill (optional; I drilled holes in the caps so I can string these on a garland)
Step 1: Drill Caps
Find a dremel or drill with a small drill bit. Hold the acorn cap steady and carefully drill two small holes at the top (to pass the string in and out).
Clean up any acorn dust so it doesn't get all over your t-shirt fabric.