Cut nine pieces of black yarn that each measure 36 inches long. Lay the pieces parallel to each other, making sure the ends are even. Tie one end of the bunch together in a knot.
Pin the knot to the cushion of a chair or couch to hold it in place. Section the strands into three bunches of three strands each.
Braid one of the three-strand sections from the top to the bottom and tie a knot at the end. Repeat with the second and third sections. Finally, braid all three braids together into one thick braid, and tie a knot at the end.
Cut at least 100 pieces of black and white yarn, each measuring 4 inches long. Decide if your tail will be all black with a white tip or a multicolored tail. If multicolored, cut the same number of each color. If tipped, cut all black until you reach the tip of the tail. The total number of pieces you need depends on how fluffy you want your tail to be. To save your hand from cramping from cutting all the yarn at once, you'll use this first 100 pieces, and then cut another batch to continue the project.
Unpin the braid from the cushion. Fold one piece of yarn in half. Hold the two tail ends in one hand and the loop in the other. Push the loop through one strand in the braid, somewhere in the middle. Pull the two tail ends through the loop and pull the ends to secure the yarn into a knot. The tail ends will hang free.
Add more pieces of yarn, working toward the top and the bottom and all around all the sides of the tail. Add as many pieces of yarn as you want. The goal is to make the tail full and fluffy. Alternate the colors in stripes, if that is the type of tail you decided on, or make it all one color with a different colored tip.
Brush the yarn very gently with an old toothbrush to unravel the yarn and make it fuzzy. Smooth the fuzz down to complete the tail.