Marbling brushes, or whisks, are made of broomstraw.  Either natural or plastic straw may be used, but plastic is much more durable, and can be rinsed out cleaner than real straw.  Many marblers still use real broomstraw, but little pieces will break off and fall into your tray, and the brushes can get moldy after a few days if they don’t dry out properly.  So plastic is better.

Take a small amount of the plastic straw (a bundle about the thickness of a pencil, or about 60 strands).  Then tightly wrap a rubber band around the end where the straws are smooth, leaving the brushy end free.  Some fuzz may come off the end when new, so rinse the ends well before using the first time.  Make one brush for each color.

Using the brushes requires a bit of practice.  Dip the brush into a color, then knock most of the paint back off into the cup so it is not dripping at all.  Hold the brush now with your thumb in the middle of the brush.  (If you hold it down on the rubber band, it will bounce too much when you tap it and paint will fly upwards into your face).  Then tap the end where the paint is, with your other hand… you will get paint on your finger, but that’s how it has to be.  Tap it on the topside of the brush, so the paint will go straight down into your tray.  You have to tap it hard.  If you have your finger under the brush and tap the brush onto your finger, which may seem easier, the paint will splatter outwards onto the wall, table and floor.