Cut three pieces of fabric into rectangles measuring 13-by-7 inches. If you choose to use interfacing to make your paintbrush holder less flimsy, cut a rectangle of this measuring 13-by-11 inches.
Stack two pieces of the fabrics together with the right sides facing towards each other, and the interfacing on top. Line the interfacing up directly with three sides, six inches away from the edge of the fourth side.
Sew the fabric and interfacing around the three sides that the interfacing is lined up with the fabric.
Open the side that has not been sewn, and turn the fabric inside out so the interfacing is now inside.
Sew the open side closed. Sew around all four sides once more.
Fold the third piece of fabric in half and line it up against three edges of the pieces of fabric already sewn together. Sew the edges together, leaving a pocket open in the center.
Measure out each brush pockets to be on the newly sewn pocket, ranging in size from .75 inches to 2 inches wide. Mark each pocket with tailors chalk.
Sew each brush pocket lengthwise. Check each pocket after sewing to ensure that you have left enough room to slide a paintbrush into each slot.
Insert paintbrushes, roll up the holder, and use your ribbon to tie it shut.