Bend the pipe cleaner around a cookie cutter to make a star shape. Twist the pipe cleaner closed at the end of the shape. Cut off any extra pipe cleaner at the end. Repeat for the second pipe cleaner so you have 2 star shapes.
Tie the thin ribbon or string around each star, then hang them from the popsicle stick.
Lower the 2 stars into a mason jar and rest the popsicle stick over the opening of the jar. Make sure the pipe cleaner stars aren't touching each other or the sides/bottom of the jar.
Pour 2 cups of boiling water into a glass measuring cup. Add the borax and stir until dissolved. If the borax won't dissolve, microwave the mixture for 30 seconds at a time, stirring each time, until the water is completely clear.
Carefully pour the hot borax mixture into the mason jar. Fill the jar until the top of the star, but no higher. (Crystals will form on the ribbon if it's below the liquid)
Leave them for 6 to 24 hours and watch the borax crystals grow!
Video
Notes
When you're done with the borax mixture you can pour the remaining liquid into a load of laundry (borax is a laundry detergent booster afterall!). Don't pour the crystals into the laundry since they won't dissolve. After you've poured the borax water into the laundry, use a spoon to scrape out the crystals - they should come out easily while everything is still wet.If the crystals won't come off the jar, fill the jar with water and microwave, stirring every minute, until the crystals are dissolved.