You'll also need a coffee cup, small basket, vase, or other container about 5 inches high with tapered sides.
Step 1: Start knitting or crocheting a simple rectangle
Do use loose stitches. If you have large knitting needles or a large crochet hook, by all means use that. I didn't have any, so I'm using the handles of my two largest paintbrushes. The ends of the brush handles aren't very pointy, but it doesn't really matter because the stitches are very loose.
In this picture, I'm pulling and stretching the work apart a bit so that you can see how loose I made the stitches.
Step 2: Attach the work to the container
Stretch the knitted square over the mouth of your container, and hold it with a rubber band while you adjust it and make sure it's the way you want it. You want it to be stretched fairly snugly.
The way I secured mine was by using a large blunt needle to weave a ribbon in and out all around the edge of the knitted piece. Then I tied it tight and removed the rubber band.
Now poke your paintbrushes down right through the knitting, to the bottom of the container. It will hold them apart and keep their bristles from touching each other.