Cut slits along the top edge of an old flannel sheet with a pair of scissors every half inch. Grab the sheet and rip strips down the the cut lines of each slit.
Cut a small slit at the ends of each strip of flannel sheet. Tie two strips together by joining them end to end in a slipknot with one strip making the "core" of the knot and the other strip making a "tail" ready to form another knot.
Cross the tail strip over the knot into a shape much like the number "4." Thread the end of the tail strip under the core strip and through the middle of the shape "4" then pull the tail strip taut without firmly closing the knot made. Hold the first knot down with a safety pin. Keep knotting along this line until you achieve the desired width to make the first row of the toothbrush rug.
Fold the core end alongside the first row of knots. Pierce the center of the last knot you made with the rug tool. Start knotting the second row and all other subsequent rows as described in Step 3. Cut off the end of the fabric and make a new slit and knot with a differently colored strip if you want to change the color of the rug as you weave.